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1) Look at the microphone's datasheet? Assume it is crap if it has no datasheet.
2) pass a known range of frequencies and wave forms into it snd look how it compares to other microphones to the input.
3) use your ears to decide which one makes you sound like you want to sound. Some DJs have terrible voices in real life - they thank their sound engineer for the right filtering and pick the perfect microphone for themselves.
I'd say the first test for any mic containing electronics is the noise level - try recording with high gain somewhere totally silent & see what "hiss" level you get.
A decent true condenser mic should have next to no noise; a cheap electret fake condenser such as an NW700 will hiss and probably give a fair level indication just from its internal noise.
For other comparisons, ideally you could do with a reference mic, a known good one.
Put the two side by side, record them together as stereo with a range of sounds from general music, bass, cymbals, vocals and a range of audio levels.
Then listen to the playback on decent headphones and compare the two channels.
Without a reference, all you can do is look at the datasheet as gophert says, plus look at independent reviews and see of there are specifications / frequency response curves for it in the mic database on Recordinghacks.
And just record a range of things and see if it sounds good!
If you want a pretty high quality and value condenser mic, look at the MXL V87.
Index of microphone manufacturers, linking to mic product listings for each. Currently mikes by vendors profiled, with more to come: dynamic mics, FET and tube condenser mics, ribbon mics, mid-side stereo microphones and more