to be honest, I think that pain would be more a mental thing than anything else in regards to transformers. I remember specifically the team trainwreck comic posted on here where a little bot lost his legs or something like that, and considering that they don't seem to need their legs (or any real appendages for that matter) to function, visual stimuli would seem to be the best, if not the only way to tell if they were injured, unless, of course, the were hit in the chest or the head.