I have some 35mm films, trouble is I dont know which ones are empty and which ones I’ve used and rewound! Is there a way to tell?
You can pretty much tell the difference when you rotate the spindle, especially when you rotate it counterclockwise (the non-rewind direction).
Failing that, especially if these reusable cartridges, you can just pop them open in a darkroom.
Finally, you could just send them out to a processor. If the canister is empty, I imagine they just won’t charge you.
How? I dont have a dark room, and I dont want to waste the unused ones!
It’s not clear how this occurred. Typically, new 35mm cassettes are crimped together. They don’t come apart easily and are not reuseable. People who reload their film from bulk rolls obtain cassettes that can be snapped apart, typically by holding the longer spindle against a table and pushing to pop the cartridge apart. This has to be in a dark room.
FInagle’s suggestion is fine. Turn the part of the spindle that sticks out. You should be able to feel if there’s something in there or not. If you cannot tell, just take it to a developing place and they can tell. And if they can’t tell (unlikely, I should think) then just let them develop them all. The people who do the developing handle a zillion cartidges a day won’t charge you to develop no film.
Now, if you’ve accidently pulled the leader from a new roll into the cartridge and cannot tell which is exposed or not; well, you’re hosed. But the fix is the same. Get them developed and you’ll be able to tell. If there’s no pictures on the roll, then let them keep it.
Used ones don’t have leaders, new ones do. At least thats how it is for me.
I’m not sure I understand your question.
It seems to me that you rewound your film on your exposed rolls, but left the leader out, yes? Well, to quote jsleek “you’re hosed.”
Don’t leave your leader out unless you have a good reason. And if you do, mark it by biting the tapered part of the leader off. Then you’ll know for sure which rolls are shot and which are not.
Or is the case that you have empty canisters and shot canisters? This doesn’t seem to be the case, as it’s rather self-evident which are empty and which aren’t. (Plus your follow-up questions don’t fit with this scenario.)
I’m unclear as to your question as well, and the reply of ‘turning the spindle’ leaves me scratching my head.
If you’ve got empty cassettes (i.e., ones with no film in them) you needn’t bother turning the spindle, just the difference in weight will be apparent.
If there’s film in the cassette, and the leader (a bit of film) is not sticking out, there’s no way to tell if the film has been exposed. (Unless it’s that APS crap, then you can look at the icons on the cassette.)
It sounds like you have a number of film cassettes without the leader film showing, and don’t know which are exposed and which are unexposed, right? Take them to a lab and ask for a Clip Test. If they look at you strange, tell them you want them to pull out and develop only the first twelve to sixteen inches of each roll. If there are no images on that strip of film, have them return the cassette to you with a bit of film sticking out so you can load it in your camera.
If there are images on that strip, have them process the rest of it. At least one or two pictures will be ruined by doing a clip test, i.e., cut in the middle.
For the record, IAAPP, and the proper glossery terms are: Cassette - Metal thing that holds the film. Canister - Plastic thing that holds the cassette. Cartridge - What people who use canisters to hold their dope call cassettes. 
Well, this wasn’t the clearest question ever posed on the SDMB, but if the OP wanted to know which rolls of film had been exposed and then wound back so only the leader is showing (something you sometimes do with SLR 35mm cameras if you want to change the type of film), well, usually the leader gets crimped a bit on the takeup reel and retains a bit of a curve. But this is not always the case, which is why sometimes you get pictures of a 10 year-ago wedding superimposed on your recent trip to the Orkneys. If you’ve completely exposed the roll of film this shouldn’t even be an issue – the exposed rolls are the ones with the leader completely wound back into the cassette.
Bottom-line, if you’re unsure if the roll has been exposed, send it out to be processed. If you feel you must use the film anyway, then don’t use it for anything critical. And from now on, mark your film so you know which is which.
I can’t imagine a Clip Test would be economical – there’s no way that the processing cost a lab would charge (not to mention the inconvenience) would be worth the cost of a new roll of film. At least in the U.S., film is heavily discounted nowadays due to the incursion of digital photography.