There's something terrible about throwing away liquor. But it's infinitely better than ruining good liquor by mixing it with stuff that's gone bad... and then serving that to people. The simple fact is that a liquor cabinet needs tending too like a garden... albeit a garden you only work on every few months. But now that you have some time on your hands, let's see about doing a little culling of the curdled and funky lurking in the bottles, shall we?
First Pass: We're fine, thank you very much
The plus side is that most of you liquor (and almost all your expensive bottles) are fine. This includes all you Scotch, Bourbon, Rye and Brandy. It also includes your rum, vodka and gin. If unopened they theoretically will last in perpetuity. If opened they may suffer some very slight degrading in flavour and potency, but it's not likely to be all that noticeable. Just try to keep them out of direct sunlight, which speeds up any degradation.
Second Pass: Liqueurs
On the plus side, the golden age of liqueurs is thankfully well passed, but we're guessing you still have a few kicking around: maybe some Cointreau or Triple Sec from that Cinco de Mayo party a few years back? The good news is that liqueurs that are above 35% (like the above or Amaretto) are generally pretty stable. Unopened they can easily go past 5 years and even open you probably have 3 years before they start to really go down hill because the higher alcohol % helps protect them (Green Chartreuse, for example, is legendarily long lived but it also has 55% alcohol). I's a different story with the cream-based liqueurs like Bailey's or Amarula. They can actually go bad. If opened they should be stored in the fridge and if not, I would touch them after being open for more than a year (also many have a that rarity—a best before date on the bottle—so look for that).
Third Pass: Fortified
One of the great crimes is that vermouth, that indispensable ingredient in some of the great cocktails—the martini, the Manhattan, the red hook—is probably the most common gone-bad culprit in the cabinet. That's because at their heart they're not spirits, but wines that have been fortified with small amounts of spirits. For red vermouth look for a deep, faded brown colour: it loses its vibrancy and brings a flatness and dullness to a Manhattan. Same with white vermouth and a martini: the colour turns to a dull yellow, like the beard of a creepy old fisherman, and instead of lifting the gin, it brings an oxidized flatness. Neither will make you, but they ruin the drink in a hurry. The newly popular, amaro (or amari, the plural) are a trickier beast. They likewise will degrade, but because they're generally higher is alcohol (vermouth is in the 16% range, amari more in the 25% range) they're more stable. But once opened I'd still want to deal with them within a year. They won't go bad, but their flavour profiles—which is everything in amari—will noticeably dull and flatten.
Fourth Pass: Bitters
They're fine (as long as they're commercially made). High alcohol and seriously, how would you know if they weren't?