Lifters look to be flat- - - - - -we are talking thousandths convex right?
I use a 6" machinist's scale or some other type of precision-ground straightedge and place it across the bottom of the lifter, then hold it up to a bright light. If I can see daylight in the middle the lifter is definitely worn out. You should be able to rock the straightedge from side to side a little on a good lifter. This allows the lifter to spin as the cam lobe takes up the slack from the clearance and equalizes the wear pattern. A flat or concave lifter doesn't spin, and it wears quickly.
Jerry
