first off, I think demons aren't necessarily evil, just on the side of darkness. they simply tend toward evil.
secondly, you should go through with whatever deal it was you made. if the deal was unacceptable you shouldn't have agreed in the first place.
thirdly, I would think it a bad idea to piss off a demon. You made a deal, go through with it or find a way to nullify it. don't make deals you aren't willing to keep.
lastly, as far as signs go I would suggest asking the demon to do something specific if he/she/it decides to accept or deny the deal.