Expiration dates are helpful, but sometimes stuff happens and glue goes bad sooner (or later) than you expect.
Apologies if you know this already, but a handy check for whether a protein glue (hide, fish) has still got stick'em power is the "string test": put some on two fingers, rub together for a bit as it cools and pull the fingers apart from time to time. If the glue is good, as it cools many long spiderweb-fine filaments will begin to appear as you pull your fingers apart. Do it with known fresh and well prepared glue to calibrate yourself. Patrick Edwards, the maker of Old Brown Glue, describes this here:
http://www.wpatrickedwards.com/gluearticle.htm and searching the web will yield more references.
Other simple tests: if the smell has gotten dramatically worse, time for new glue. Ditto if you see mold.
Storing any prepared protein glue in the fridge will extend it's life (for the same reasons it extends the life of foods).
Finally, re Beau's comment about not seeing any difference from hide glue: the working time is far longer. In my other life I use Old Brown Glue for very complex furniture glue ups, it's much less stressful than any PVA. The flip side of long working time is that the clamping time is generally considered longer, though I haven't tested this to push the envelope.
Frank Ford suggests that liquid hide glues are a bit more prone to creeping. They clean up a bit differently as well--the squeeze out from liquid hide glue doesn't become rock hard for ages. This makes one wonder if the glue dries rubbery, but Patrick Edwards says that the glue line is rock hard, it's just blobby squeeze out that remains softer...an clean up advantage over hot hide glue in his view.