Determine how old the paint is, if you can. Oil-based paint can be good for up to 15 years. Latex paint can keep for up to 10 years if it hasn't been frozen and thawed out over and over.
Brush latex paint on paper -- an old newspaper is fine. If the paint is lumpy, it may not be usable. Opinions differ on this point. You can throw the paint away according to some professionals but others say to strain it through a plastic paint strainer.
Stir the paint. If the paint will not stir because it has hardened in places, whether oil based or latex, it can't be used. Sometimes paint will have a thin film over it. Take the film off and then stir the paint to see if it's hardened on the sides or bottom.
Stir paint that is not hardened. If you have trouble mixing it, you can take it to a paint store or department and ask them to remix it for you. If it mixes well, it's probably still good.
Smell the paint. Let your nose tell you how good your paint is. If any kind of paint has a foul odor, it isn't usable no matter what you do to it.
Keep paint from being ruined by using and storing it properly. Paint from a second container so paint doesn't dry on the can and settle back into the paint. It also won't be contaminated by things picked up by your brush. Keep the paint can covered after you pour the paint out of it.
Store paint safely away from children in a place where it won't freeze. Repeated freezing and thawing destroys paint.
Dispose of paint properly. It's best to use it up rather than throw it out. The bottom line is that it's usually safe to use your old paint -- it takes a lot to render it useless. If it's completely unusable, let it harden and then throw it out. Check with your community's sanitation department to see if there are special criteria to follow for disposing of paint.