I have busted out jars months later and still had success.

Just look for myc piss (a yellow liquid that is waste product of mycelium)  too much of that is nasty...  also smell the jar.  A really sour smell may indicate bacterial contamination.  Any mold contamination should be readily visible.

Also, shake the jar (if grain) 24 hours before doing anything, if healthy the culture should recover, if not healthy it will die.