Praying to God to make certain things happen makes those things more likely,
The only plausible explanation of the increased likelihood is that God makes some prayed-for events happen.
Therefore God exists.
The hypothesis that prayer works can be tested, just like any causal hypothesis, for example:
Echinacea helps prevent colds
Magnetic fields reduce pain.
The gold standard of causal tests is the randomized control group study, where subjects are randomly divided into a treatment group, receiving the agent being tested, and a control group, receiving a placebo.