I recently came across an excellent online ear training service.
It trains your perception of various types of differences in audio quality. It starts at a Basic level with each step having a training section and a test section. The test then progress in difficulty where the margin of difference keeps decreasing. There is no way to cheat. If you sign up and log in, the site will remember your result and where you left off.
At a Basic level the differences are quite large – 10 – 12 dB [deciBels] and most people somewhat familiar with sound will find the Basic level test fairly easy to do. However, as you progress up and the gradient increases, at the Gold level the differences are 1 dB or less.
Highly recommended for all people involved in music, sound or audio!