I put a clipboard on mine and use my iPhone with a level app, or a real level whenever I manage to find it.

I've found that even if it's tilted up or down a 0.5 degree I'll notice it pretty quickly when riding.

I just found this weekend that it wasn't level side-to-side, though, which explains a few times. Turns out when torquing down the seat clamp it was tending to move the saddle to one side. D'oh!