I'd take it to the paint store and ask them to shake it for you. Then open it and see how it looks. Try it, no loss if you decide it's not any good.

I'd open it first, and look and smell. If there's mold floating on top, scoop it off, the rest might still be okay if it smells right. If it smells awful, like sewer or sulfur, then it's shot.
If there is a thick layer of solids fused to the bottom, or looks like cottage cheese, no amt of shaking will fix it.
Casey