Existing is evidence of its own. If you exist, you have evidence that you exist, do you not?
The simplest piece of evidence would be "look at him/her/it, there's god".
I have evidence that I exist, but others are not guaranteed to. The god by definition has evidence of his own existence, courtesy of "I think therefore I am". There's nothing that says that anyone else is guaranteed to be able to witness evidence of god though.
We're talking about an omnipotent being that is beyond the rules of our universe by definition. I don't think it's rational to apply the rules of this universe as though they apply absolutely to all beings, especially of higher powers. You can make assumptions, but recognise that they are exactly that, assumptions.
You're making the assumption that anything that exists can be detected in some fashion. That's a very reasonable assumption within our universe, because that's how things work, or at least how they appear to work in every case so far. You're extending that beyond our universe, and that's fine.
Agnostics don't.
This harkens back to the discussion of axioms in the rights thread. You've chosen to assume that evidence and existence are always linked. But if you're talking about a being that's outside the system, you can't assume that all the rules of thumb that you use to describe your daily life hold. There's no particular reason why an omnipotent and omniscient being need leave any evidence at all.
I think this is where you're falling down. You're talking about a being that can do literally anything. You're trying to define it by what is physically possible, and that's not how gods work. Gods make the rules, they may be bound by them as well, but there's no logical reason for believing that's necessarily so.
If I'm playing Gran Turismo as a player, I'm bound by it's rules. If I'm a programmer with access to the code of Gran Turismo, I can do pretty much anything I like. If I want to have a race online with other people, I can probably make it so that there is no in-game way that they can detect me. They'd have to be outside the game to be able to detect what I was doing. And then I could avoid that detection by being one more level further up, a being beyond the rules of this universe. Repeat ad infinitum.
It's a foot race with the observer always one step behind. It's always possible that there's a being one more step beyond your ability to detect them.
This isn't terribly useful reasoning for daily life, it's awfully reminiscent of navel gazing. But so is a lot of philosophy and religion. As far as I can tell, the principle that it is always possible that there is a being beyond your detection is logically sound. If you label that being as god, then the statement "there cannot be evidence of gods" works too.
Anyhow, I wasn't saying that I agree with what Mr. Sheldrake is saying, for example when he says that one dogma that mainstream scientists believe is that everything is constant, claiming that the speed of light has changed over the years, but I think that this is because of the development and improvement of the equipment we use to measure the speed of light and that it has remained constant.
There is a lot of evidence that the speed of light is a constant. Scientists don't believe in this in the same way that Catholics believe in Jesus, they simply accept that evidence at face value.
There are circumstances that would be compatible with our current understanding of the universe under which the speed of light would be changing over time. It's very hard to detect, and to my knowledge they haven't managed to do so to the precision that would be required to overturn such a well established theory as the constancy of the speed of light.
Laypeople have this misconception that scientists learn one thing and then stick with it to the grim death. That may be true of some bad scientists, but a good scientist is aware that all they ever have is a best approximation that fits the circumstances. Some of the approximations have been refined so much by so many people that we're really pretty damn sure that they're as accurate as current technology can make them, and it would take a major upset to seriously overturn them, although minor tweaks happen all the time.
The crucial thing about science though, is that anything that it claims has at least one thing that, were it to happen, would prove that claim completely wrong. Generally, scientists find it most efficient to come up with an idea and then try their very hardest to break it. This is why people find it tough to attack scientific theories, because these are the ideas that have survived having lots of very bright people trying as hard as they can to prove the idea wrong.
This is the problem with things like morphic resonance. If there's no way for something to be wrong, then there's no sensible fashion in which it can be said to be right either.
But all is not black and white, things are mostly grayish, a mix of god and devil. 
I'm not above error. 
