Buying used crash pads

I'm considering buying a used crash pad. What criteria would you use to evaluate whether or not a used crash pad was solid? I

open it up and look at the foam, if the foam is in good shape your probably good to go.

+1 what Jacob said

Look at it all you want. You will never know how many naked hippies have been on that thing.

Stand on it when open and see how much you compress the foam to the ground. also try on the backpack straps, if it has some, and see how comfortable it is to carry.