For some more scientific benchmarking you can use -benchmark option for mplayer and measure time and cpu load for different mplayer options and different types of video

If you want to check whether some video can be played smoothly, add -noframedrop option, with this option mplayer will complain when it is not fast enough to play this video without dropping some frames.

By the way, probably 2-pass encoding is not the best choice for smooth playback on the resource limited device. It will make some parts of video use lower bitrate than you specified, but some parts will use higher bitrate and may become too heavy for cpu. So if you don't care too much about file size and want a smooth playback, 1-pass encoding is a way to go.