Taken from my trusty Mixing Engineer handbook:
Things to listen for in a Monitor:
EVEN FREQUENCY BALANCE
While listening to a piece of music that you know well, check to see if any frequencies are exaggerated or attenuated. This is especially important in the crossover area (usually about 1.5 to 2.5khz). Listen especially to cymbals on the high end, vocals and guitar for the midrange and bass and kick drum on the low end.
FREQUENCY BALANCE STAYS THE SAME AT ANY LEVEL
The less the frequency response changes as the level changes (especially when playing softly) the better. In other words, the speaker should have roughly the same frequency balance when the level is soft as when it's loud.
HIGH OUTPUT LEVEL WITHOUT DISTORTION
Be sure that there's enough clean level for your needs. Many powered monitors have built-in limiters that stop the speaker or amplifier from distorting, but also may keep the system from getting as loud as you may find necessary.
Reason I wrote all of this is because this to me is key in finding the right monitor for yourself, and for whatever monitors you may currently be using to help get accurate mixes. Everybody is going to have their own favorite monitors, and there are many choices. you can use just about any type of monitor IF YOU KNOW the limitations and or strong points. Take your favorite CD's that you really know well and go to the music store and audition the monitors. That would be my recommendation.