It was mildly impolite of you to not bother learning and adhering to the rules but ego te absolvo.
You did kinda screw up your patriotism check, btw, but for some reason that kind of thing is encouraged around here. Go figure.
You only need to confess at reconciliation for mortal sins. It is fine to confess venal (minor “everyday”) sins directly to God in prayer.
You’re talking about a Mormon temple ceremony, which yes, you do have to be Mormon to attend (or to enter the temple for any other reason). There are plenty of Mormon weddings not held at the temple which would be welcoming to any (assuming you’re welcome to the wedding in the first place.)
Inside secret: Don’t forget the nut tap at the end, it’s how we know you’re really Catholic for reals.
Cradle Catholic here. Though not any more.
No one will stop you, so don’t worry or give that a thought.
As for communion, “take communion” is the way Protestants say it. Catholics say “go to communion.” “Are you going to go to communion?” Also, you can take the wafer in your hand or open your mouth so the person with the ciborium (golden cup holding all the Jesuses) can put the wafer on your tongue. You don’t have to cross yourself afterwards. You may, but no one will notice or care if you don’t.
I don’t think people genuflect all that much any more. I don’t think I could do it now to save my life. 
If you wind up in an informal receiving line as you leave, shake the priest’s hand and say, “I enjoyed the sermon (or homily), Father.” If he asks about you, say you’re just visiting. Or else go out the side door. 
My late husband and I used to attend a Methodist church in the small town where we lived and I just moved from. Before communion, the minister would step to the center aisle, face the congregation and say, “This is the Lord’s Table. You are welcome, whether or not you are a member of this church or any church.” I still get goosebumps when I remember that. That’s the way it should be.
the ushers are gatekeepers skilled in profiling. They are Mackerel snappers old school.
Which makes the OP who? The Keymaster?
Sure, someone could attend & they might even fake it through the communion line. Still, isn’t there an easier way to get a bite to eat?
Back in the day when I was a student who traveled a lot, I used to attend Mass at several different Catholic churches without formally being a member of any of them. Not once did anyone comment on my presence. I agree that they will be happy to have you there as long as you don’t create a scene. They don’t do anything to “check” if you are Catholic or not. Lots of non-Catholics go to mass with their Catholic families after all.
In fact, when I got married, my husband (who was never baptized in any religion and is open about being an atheist) was very anxious about talking to priests about getting married in the Catholic church, but the priests were very understanding about it. They didn’t give him a hard time about being an atheist at all. They expect the non-Catholic spouse to agree that the children can be raised Catholic, but unbaptized non-Catholics can marry in a Catholic church if they are marrying someone who was baptized Catholic.
You should anticipate that right before the Mass begins you will be asked to do “the sign of peace” (shaking hands with your neighbors in the pews). There are several points in the service where you are supposed to stand or kneel - just watch what everyone else does. Or, if you have a physical handicap and can’t get up and down, you can still just sit quietly. Nobody will care.
Under ordinary circumstances (apparently there are some exceptions), you are not supposed to take the Communion if you are not a Catholic even if you belong to a different Christian church (sorry, drewtwo, but your friend who told you to do it was mistaken), but since they don’t “check” it’s not like anyone will stop you.
At the end, you will often see that people start to leave after Communion. That’s considered a bit impolite, though. You are supposed to wait until the priest says something along the lines of “The Mass has ended. Let us go in peace”.
A few points of clarification from someone who actually goes to Mass –
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The Sign of Peace comes right before Communion, after the Our Father, not at the beginning of Mass.
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People who distribute Communion are absolutely called “ministers” – Eucharistic Ministers, unless you’re talking about the priest or deacons. They also distribute the Blood of Christ.
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Use the term host rather than wafer or (God forbid) “cookie” (though I’m not insensible to the fact that this was probably used pejoratively.)
And no you will not be judged or asked for credentials.
Also, don’t talk or joke around in line. I have seen a priest take someone to task for that.
Only if you were first in line so you hadn’t watched the others to see what to do. But it’s not rocket science.
The priest holds out the host, says “The body of Christ”. You say “Amen” and either stick out your tongue to have it placed there, or hold out your hand to accept the host and then administer it to yourself. Some places may give you a sip of the wine, too. It’s been awhile for me, so maybe things have changed.
Now, in the old days when the priest said “Corpus Christi”, and you replied “No, but my aunt Mabel lives there”, that would be a dead give-away. 
Nitpick: “venial”. A “venal” person is unscrupulous and open to bribes - something you urgently need to see a priest about!
You can think of “venial” sins as being minor the way that “menial” labour is small-scale and everyday.
I’m confused. I also grew up Lutheran (ELCA; Rocky Mountain Synod) and learned from our admittedly very conservative pastor that it was a Sin to take communion at a Catholic mass because of the very issue of transubstantiation vs. consubstantiation. As an adult, I refrained from taking communion at Catholic masses because I felt it would be disrespectful to the priest to represent myself as believing what he believed about the Host by participating in communion. A Catholic friend of mine took great umbrage when I didn’t take communion at her wedding because she thought it didn’t matter what I believed; we didn’t consult the priest because it didn’t come up beforehand.
I became a Methodist after I married, mainly for the sake of convenience, and I liked that, too. Of course, that welcoming attitude is an effect of the Methodist belief that Christ’s actual body and blood are only symbolically present in the Host.
Now that I’m an atheist and don’t believe in any part of what communion represents, I don’t think I’d participate in it at any church, mainly out of respect for the people who do believe.
You’ve watched the scene at the end of Indiana Jones and The Last Crusade when the bad guy drinks from the wrong Grail? Don’t man, just…don’t…
You don’t need to be Catholic, although in theory you should be from one of the Churches which believe in transubstantiation. And there’s always the “in good faith” clause: it’s not a transgression if performed with good intent.
There’s a specific way to receive the host in your hand. Don’t just go up there and hold it out like you’re asking for change. And better yet don’t attempt it if you’re not Catholic. It’s disrespectful at minimum, and a lot more to Catholics.
Yes you can go in, but should not make like a Catholic. Why would you want to?
No they won’t.
IMO:
If a person tries to take part in things such as confession, communion, or any other thing that would, to an adult, be akin to trying to deceive others that you are a Catholic or in any other denomination whose place of worship you might be welcome to enter, then try to act as though you are an actual member, you have proven to be a person only worthy of contempt.
I can’t think of a honest reason to try to so ** in this country & in this day & time.**
My wife is a Catholic and I’m an atheist. I’ve attended mass in four different countries (U.S., Australia, Canada and Japan) and in two different languages (English and French) with her. I’ve had the feeling that anyone is welcome to attend mass, and there’s no problem with staying in your seat and not taking communion. The only real participation required is to shake hands with your neighbours when the congregation is offering each other the sign of peace – you just say “Peace be with you” then.
We have a relative who as a teenager started attending a nearby Catholic church. (Several Catholics in the family including a nun so it wasn’t a random choice.)
After a few months they started to pester her. She couldn’t attend anymore unless her parents joined. And by “joined” they meant starting contributing $. The parents had no interest in any of it let alone paying money, so she had to quit.
We’re not talking about a 5 year old being dumped on them for cheap babysitting.
Completely shocked by the whole sad thing. Some parishes are apparently run by jerks.