For most finishes in indoor situations, I rely on a practical test:
Get a small sample of whatever you're going to apply it to. Prepare that sample as you would normally.
Open the finish. Stir it thoroughly (but avoid mixing air into it), getting ALL the solids back into suspension. If it seems unreasonably thick, add small amounts of the appropriate solvent (water, for latex paint) until it's back to a workable consistency.
Apply the finish to the sample.
Give it a reasonable amount of time to dry. (Check the label.)
After that, if it's still sticky, or if it rubs/peels off, or otherwise doesn't behave as you expected it to, chuck it. If it gave you a clean, hard finish on the sample, and you like the look of it, you can probably go ahead and use it.
I've used decade-old (at least!) latex paint left over from a prior owner to touch up around the house. Outside of the problem of the can sometimes having rusted into the paint (added iron oxide pigment, not well distributed?), it's generally worked reasonably well. I wouldn't use it on something I was building for someone else, but for my own place... hey, worst comes to worst I get a new can and paint over it.