Most of the chemmy stuff is fairly stable. Organics less so- watch for changes in appearance, smell. Biologicals ( ie voodoo, pihrana) can go bad fairly quickly.

The big things to watch for are fungal contamination, precipitates, weird colloidal suspensions ( free floating snot), and any change in appearance,smell, viscosity.

Implications? Weird pH behavior, lockouts, lack of general vigor- it can be hard to diagnose.