My buddy cleaned out his liquor cabinet today. Someone probably gave him this bottle 30+ years ago. He never opened it.
Can I assume it’ll be OK to drink it?
Not guaranteed, no, but it either will be or it rather formidably will not be. And it should not be un-OK in the sense of dangerous to him to smell and taste a small sample.
If it was never opened it should be just fine, so long as it wasn’t exposed to excess heat or anything like that.
Even if it’s off, it should be safe to sip and test.
YMMV. I am neither a doctor nor an alcoholic.
Old alcoholic beverages can be harmful? 
I’ve always thought they were like honey, which you can eat when it’s 3,000 years old.
Hmm… I have a 30 year old bottle of Pear brandy with a pear in the bottle. Wonder how that would taste?
A handful of us bought bottles of champagne when we started medical school. Most were kept in refrigerators for 4 years and opened at graduation. They were all rancid.
Sure, but some wines are *meant *to be drunk young. Others will mature well in bottle, especially port (vintage usually spends only a few years in wood then is laid down long-term in the bottle). Spirits are usually stable but no longer maturing meaningfully once they’re bottled - the time spent in the cask is what flavours them. Champagne versus brandy is not really a meaningful comparison.
Brandy is not meant to be stored in the bottle that long. It’s also a shitty brand of brandy as well. Use it to clean your sink.
were they kept upright? If the cork dries out, goodbye wine.
Yeah probably so, in the doors of various fridges.
Not Champagne; the effervescence keeps the cork moist. In fact, a lot of authorities believe that storing Champagne upright actually is better than on its side.
A decent Champagne should last for 4+ years just fine. My guess is that the Champagne USCDiver and friends bought was low-quality and not great to begin with.
interesting, I was not aware of that.
I couldn’t tell you about Korbel, but in general, brandy should lastfor years. Assuming it’s been looked after.
Doesn’t this increase the chances of the wine reacting with the cork? Or do they have to be in full contact with each other?
I am assuming it hasn’t. That’s the safest default. Not like it’s going to kill you, but life is too short to drink inferior booze. 
It should be fine. I can’t imagine there being much of anything in it that could go off. The only liquors or liqueurs I’ve had go off on me are ones that contain cream or milk. They end up curdling/souring/solidifying. Just give it a sip and see. You won’t die or anything. And (if it passes the taste test) make yourself a nice Wisconsonian brandy old fashioned sweet/sour/press/etc link here. Sweet is topped off with 7-up. Sour is 50/50 or Squirt. Press is half 7-up, half seltzer. Or you could just do all seltzer. It’s not what the rest of the US calls an “Old Fashioned,” but it’s a reasonable Midwestern drink. And Korbel is the usual brandy it’s made with.
Brandy is a spirit not a wine. It is wine that has been distilled which makes it pretty much shelf stable. Another way to think of this is if it were an unopened, 30-year-old bottle of vodka or scotch, would you have the same concern? Probably not, right?
As such it should, in theory, taste just fine. However, as someone else mentioned, spirits really don’t improve with age once they have been bottled. That typically takes place while aging in oak barrels. And since Korbel is not really a high quality brand it is doubtful it will be anything very special.
What I said above should apply to your pear brandy as well. As I don’t know the producer I can’t vouch for how good it is but chances are it could be pretty tasty since the cheap stuff doesn’t have the fruit in it all that often.
Wine coming in contact with the cork doesn’t automatically cause a reaction to happen that will harm the wine. I assume you are asking about “corked” wine. When a wine is corked it is typically caused by the prescence of a chemical called 2, 4, 6-trichloroanisole or TCA in the wine. This can be caused by contaminated corks but oak barrels used for aging can be the source as well as oak beams in the winery or parts of the wine press. My point being, it generally happens during some part of the production process and not while it is being stored in your wine cellar. There are other faults that can happen due to improper storage but the nasty mildewy smell that is typical of corked wines almost always happens in the winery.
Yeah. A 12-year old bottle of scotch that’s been sitting in the liquor cabinet for the last 18 years is still just a 12-year-old bottle of Scotch, as far as aging is concerned, not a 30-year-old one. I got married four years ago, and I finally broke open a 12-year-old bottle that I had from my travels in Scotland in 1996 (which would, if aging worked that way, a 27-year-old bottle of Scotch.) It still tasted like a standard 12-year-old Scotch for the most part (of course, I can’t say for certain, as I didn’t have a contemporary 12-year-old bottle to compare it with, but it didn’t develop any amazing flavors or anything obvious from an extra 15 years of shelf storage). But it was nice for the sentimental value.
That doesn’t make sense to me. The “effervescence” is CO2. CO2 doesn’t keep anything moist.
Oddly, the idea is that the humidity is higher because of the carbonation and that keeps the cork moist.
I’ve also been told that for short term storage keeping the bottle upright is fine, mostly for convenience sake. However, any long term storage of fine wine, still or not, should be with the wine stored on its side. BTW, minimal exposure to light and proper temp are very important factors.