There are many ways to do this, even for an 'objective' assessment.
To remove too much subjective influence, find a suitable test subject, put your camera on a tripod, and step through each of the candidate lenses in turn. Make at least three different shots with each lens (wide open at f/1.4-f/2), medium aperture (f/5.6), and stopped all the way down (f/16 should be available on all 50 mm lenses presumably).
If the lenses don't provide EXIF, ensure you keep copious notes of lens/settings to consult later.
Go through the test shots and look for chromatic aberrations, signs of flare and veiling, image contrast, colour rendition, and the way out-of-focus elements are rendered. Sometimes the rendition of the unsharp zones on either side of the plane of focus can be drawn very differently.
