The main thing when it comes to checking out a lens is to see if there are any scratches on the glass (on both sides of the lens). If a lens has autofocus, then make sure that works and how fast it is. Look for signs of it being dropped, as that could cause misalignment on things internally. I’ve heard it said that if a lens hasn’t been taken care of, there could be mold inside due to condensation, but I’ve never found that before. Also check to see if it has “lens creep” or if there are any scratchy noises you zooming in and out. Those are the things I know to look for!
