Sorry if I'm catering to a whim, but since it looks like VfF is not interested in any other procedure than kidney detection, would this be workable?
One curtain with a hole cut out at the appropriate height.
One package of say, six identical cheap white T-shirts.
2 volunteers ?KG, one Missing Kidney Guy (MKG, heads) and one Two Kidney Guy (TKG, tails).
One openbacked chair
One coin
Paper and pen
The two volunteers each don one of the white T-shirts. If they want to change after one or two viewings, they can take a fresh one.
1. They toss a coin, record the outcome, and the appropriate ?KG goes to the curtain, and sits down with his back to the hole in the curtain.
2. VfF comes in, zooms in on the kidney area and records 1 or 2 kidneys.
3. VfF exits, then ?KG exits.
4. repeat from 1 until there are 10 recorded 1's or 2's.
5. Compare notes.
6. The test fails if........ (Statistics for the number of correct readings).
Femke