apparently what you do is put them in water. if they sink they're really
fresh. if they float a little then they're getting old and if it bobs on the
surface it needs throwing. it's because of the porous shell letting in air
as time passes.

in the long run they're not expensive fresh ones are better for all types of
cooking and it isn't worth the risk. just go and replace them after a couple
of weeks at most.