You've found a piece of pottery at the antique mall and it looks pretty good, but you're not sure if it has a hairline crack or just a little crazing.
How about giving it the sound test? Flick the jar with your index finger and see what kind of sound it makes.
Does it ring like a bell or is it a flat sound? A crack in a jar (even tiny ones) will make a very flat sound. THUD
Whereas a "good" jar will ring clear like a bell.