Hi..
If you are concerned about cracks: they will be ovious on the inner part of a rim, generally - as that's the relatively weaker part of the rims..
If you are worried about buckles or flat-spots, these are sometimes visible to the naked eye. However: the better way to check, is to take all the rims somewhere independant and ask them to put each of the alloys onto a wheel balancing machine - to ensure they are round; true and straight, - with no buckles or flat spots on the inner or outer parts of each of the rims..
As for, how you can tell if they are good reps'.. Hmm, a difficult one, - but the best of luck in your quest however..
Out of interest, - have they been refurbished and wet painted or fully refurbished and Powder Coated?
Things to check for: decent / new, Centre Caps and decent / new M Badges - both if fitted?
Cheers..