cylinder bar replacement.. when?

Having never replaced cylinder/rasp bars or held/seen new ones, what should I look for to tell or know when to replace them? Silly question, but I am relatively new to these combines.

Gleaners? If you look down the length of a new one,they have a high point down the center of the rasp and a rather square corner at the leading/trailing edge.When the leading edge gets rounded off they say they wont grab and pull material through.I'll say that the peak down the center is just as important for pinching and pulling stuff through as well.When the top is rounded off and the leading edge is rounded off you'll have a difficult time getting it to take anything through although they still don't do too bad in corn. Soybeans,forget it!A little hard surface on top of a rounded leading edge and a short bead across the center of the peak will help a lot.