obvious one, check for leaks once co2 is in.
check it's shooting in single and double action.
check to make sure the mag is indexing correctly.
and most importantly check that the bolt that holds the barrel section if there otherwise you'll look bloody stupid when you thumb the lever to put a mag in and it goes flying off lol
one last thing, always have a good look at the screw/bolt heads for any signs that they have been tampered with, if they are marked start asking questions as to why.
hope this helps