Here are some things that I would consider strong evidence:
1. If praying to a particular god caused something spectacular and inexplicable to happen (like, say, allowing people to re-grow lost limbs) but it didn't happen when people prayed to other gods.
2. If an ancient holly book (or any other religious source that was supposedly "from god") contained a lot of specific, unambiguous information or predictions that could not possibly have been available to human authors at the time the book was written. Things like "There will be a powerful earthquake in Geneva on March 3rd, 2017. 53 buildings will be knocked down." I'm not talking about things that can be "interpreted" in a certain way, or things that are generic enough that they could end up being correct through chance. Also, most or all of the predictions would have to be correct; a few correct predictions out of many failures would not be very impressive.
2.5 If follower/priests/holly people of a particular religion could make predictions as above.
3. If the followers of a particular religion were magically immune from physical harm - if bullets stopped suddenly in front of them for no reason, they never got sick, they could walk through intense fire without harm, etc.
That's just off the top of my head.