Larry pretty much nailed it there.. when channels are aggressive, they will smack just about anything we are putting down there or bait. It seems they tend to hit the smaller bullheads more often, but they will take anything available at times.
I have caught flatheads before that did considerable damage to the bullheads. Flatheads can bite pretty darn hard when they get bigger, but nothing compared to a big channel cat.
Its only opinion, but I do believe that a person can often tell what type of cat crushed the bait. Often there is a single crush wound on the body. Flatheads have a much wider jaw than channel cats, a narrow impression to me suggest channel, a wide impression(more than 1″) suggest a flathead. Flatheads teeth are much more coarse than channel cats also. Flathead teeth are similar to very coarse 36 grit sandpaper and can rip flesh pretty good, channel cats tend to rip the flesh more by pressure and it will often result in a clean looking tear at the edge of the bite depression.
Did any of that make any sense?