ROSE: Well, he's got two hearts.
JACKIE: Oh, don't be stupid.
ROSE: He has.
JACKIE: Anything else he's got two of?
(The Christmas Invasion, 2005 Christmas special)
So is there anything else he's got two of? Or to put it more generally, what are the physiological differences between a human and a Time Lord, other than the number of hearts? How can you tell, just from examining a Time Lord's body, that he - or she - is a Time Lord?
Let's say you can't kill the person to see whether they regenerate, nor can you wait hundreds of years to see whether they die of old age. All you can do is a one-off examination of their body. You have all the equipment you need: stethoscopes and so forth. So that you can see the whole of their body (as well as to exclude the possibility of identifying a Time Lord by their dress sense), let's assume they're naked: