The best evidence for the bomb being fake was the way Cooper handled it.
If you're handling a live bomb that could blow you into pieces, whether you detonate it or not, you'll be relatively careful with it..i.e. you'd much rather blow yourself up on purpose, rather than "Oh shit!"
The idea of Cooper with a bare wire with his hand in the briefcase, and especially getting up with the briefcase horizontal with his hand in it, ready to detonate by touching something he can't see?
That's ridiculous. If you have a bare wire in a briefcase, and you can't see what you might touch that would detonate, and you're jumping around up and down...you're basically allowing randomness to control things.
So there's really no way the bomb was real.
You wouldn't build a real bomb, and stop when it came to reliable detonation and decide "Oh, I'll just have a bare wire for the switch"....Someone could push your arm, and BOOM! That would be a waste of a good bomb and the rest of your Raleighs.