Hmmm, this is how I learned this, back in textile class:
A microcheck is any woven check, plaid, or pattern that is small in scale. Can include glen plaid, windowpane plaid, Prince of Wales plaid, herringbone, houndstooth, etc.
Glen plaid, windowpane paid and POW plaid are the three most common kinds of plaid- they are very specific patterns, but can be large or small in scale.
So, a glen plaid can be a microcheck, but isn't always. (I was under the impression that the CM microchecks were a glen plaid, but I'll have to check to be sure!)