[dtxtremeiiispecial]

I guess other ways I know I can play are:

I can hear myself keeping a good beat. I can hear myself playing a good beat and good fills to a song. I can feel the rhythm while I keep the beat.

Also, instead of asking, "How do I know I can play?" I would like you to tell yourself, everytime you do something well, no matter how small, I would like you to say, "I can play,"
"I can play well," "I'm getting better," "I'm improving," "Wow, that was really good!", and so on.