The lifter face shape has been mentioned. other dimensions can be out of whack as well
in a box of 12 or 16 you can also find one with a body that is under sized i.e well out of tolerance, when oiled just falls through the lifter bore
on a six if i can put it in with 1 hand and get my hand under to catch it in good time i'm happy
if it fall through onto my mat in double quick time, i'm not happy, it needs to be measured to provide proof, and there is a long wait for another to arrive from the supplier
if it never falls through i'm also not happy that needs investigating and it may be more suitable in another hole.
if i turn it and it then falls through is it dingged? is it as cylindrical as it should be? or is the bore scored or has a hangup at the oil gallery catching the oil band on the lifter.
did i choose a lifter with an appropriate oil band position for my open gallery style of block?
if the originals had a narrow band do these wide oil band versions cause a problem?
lifter face can be checked on flat plate or look for light with it against the side of another lifter
lifter size can be measured with your electronic calipers, they are accurate enough to show a bad one.
lifter could be AMC in which case the seat for the pushrod may well be at a different height in the body from a mopar one both types fit but you won't spot the difference easily, use to your advantage rather than get caught out, this could be the difference between a pushrod rubbing somewhere or not.
and indeed oiling through the pushrod or not
All just stuff I've picked up from people way more experienced than me that i've found useful when trying to avoid issues
Dave