If you are doing this inspection work lol on your own the easiest way is to have Taz on a slick surface (counter top) where he/she can't get grip with back feet then get down to eye level and gently lift her up by the tail just enough to see under her belly. Really shouldn't make a huge diff though unless you plan on getting a buddy but even then we have deducted from the evidence or lack there of that if it is a male it is neutered so you could get either sex as a buddy. Problem would be if you planned on breeding.
I foster wild opossum for a wildlife center. So I care for and rear injured or orphans then release them when they are old enough. I also foster squirrels and last yr some skunk out of my home .. All the other animals are at the center ie racoon marmot etc.