Well you're getting ahead of yourself now by wanting evidence.
We have the claim "An omnipotent being exists" (called God for purposes of discussion).
Before we go off trying to test this claim (thus gathering evidence), we need to look at the claim and see if it is testable. It isn't.
And yes, it can be tested. All of Christianity (and other religions) believe in prayer. There has never been a successful test of prayer. Indeed, every prayer asking for anything is a test. They all fail.
That's testing prayer though, not the existence of God. Prayer is indeed testable (or at least the claim that prayer can bring about results in the physical world). On paper, anyway. The problem is that you're dealing with an abstract concept - maybe God has His reasons for not answering prayers. Maybe He only doesn't answer them when under examination, because He doesn't want to be so obvious, preserving free will. You can always wiggle out of it because you have something that by definition can do anything.
It's sort of like the pretend fights you had as a kid.
"I shot you!"
"Nuh uh, I dodged it!"
"Yeah well I have heat seeking bullets you can't dodge them!!!"
"Yeah well I have diamond armour!"
"Yeah I have diamond bullets!!!!"
Your shot will never get through, sorry
So the concept of omnipotence and God are unscientific (hence why I don't believe in them) - they lie outside of the realm of science.