When they are stopped after curfew by the Mohawks, Hawk jumps at the window and orders them to give them 50 bucks to go on, and they accept credit cards. When saying this, you can see the dividing line between his real skin and the fake head cap that supports the actor's fake hair.
When Billy is fighting evil Jimmy, he says "Shadow demon, you shall not pass." Look behind him. The actual Double Dragon arcade machine is behind him, which Jimmy then punches.