Modernizr has a test for video autoplay (it hasn't been released on the modernizr.com website - just the github repo. What it does is check to see if a bundled base64-encoded super-short video is playing, and if it is, it returns true.
Most of the time, the best thing you can do in this instance is to check for mobile browser useragent, and fall back to a static image when it is encountered.