if its one of the common wide agnles, theres probably a lot more scientific testing done by the testing websites that u can refer to.
but if u do need to test... its best to do it indoors because u have 100% control over lighting.. if u see DP review, they set up a umber of different items on a table and shoot that..
some people use newspaper taped to the wal.
the importnt things to note -
tripod mount the camera.
use mirror lock up/ cable release if u have it.
take 2 or 3 exposures at each shutter/aperture setting and choose the best one , to eliminate any misfocused shots. this ensures u ahve the best picture at each point of comparison.
the normal comparisons are normally made wide open, mid focal length and maximum focal length (assuming its a zoom).
for F stops, again, compare wide open, one more at F5.6 or F8, and another say at F11 or F16.