may i ask some question?,
a:the register is electrical or waters?
b:how thick is the wood floor?..if floor is to thick will need lot of thermal diferential.
c:the primary floor is concrete? if so, is there some access from below where you can heat/cold up?
well, if the register is electrical, maybe with some devices like wirefinder's will be easiest.
the wood is very good insulator (does not transport energy efficently) so, any way to try to find hot or cold from the other side will be difficult if the wood is too thick.
please let me know the final of history.