Hi Pam
wheels are pretty easy. Give it a spin - if it wobbles, it's crooked. If a bike is really really well used, as in the wheels are used up, you will see a considerable valley in the rims. I'm going to have to get photos of my husband's rims, come to think of it, why hasn't he gotten new wheels yet? hm.
ok, what else.
What is fun to do is to get a bike and then take it to a bike shop. If it's a good bike (not a walmart bike) they will happily adjust the brakes, true your wheels and change your chain... for less than $200.

ok, back to the frame. Yes, you can see cracks in frames. you can eyeball and see things that are bent.
good luck on your hunt..