Either transmission will work on any 7M, but they use different flywheels, starters, clutches and are mated with different differentials. The R154 has an inspection panel on the passenger side of the bell housing near the bottom. The W58 has no inspection window at all. The R154 is also a lot larger but unless you've seen both in the past (in which case you probably wouldn't be asking) that info gives you nothing to go on. The W58 won't hold nearly as much power as the R154, up to 350 or 400 if in very good shape.
R154:
http://www.cygnusx1.net/Supra/Librar...ection=MT&P=49
W58:
http://www.cygnusx1.net/Supra/Librar...Section=MT&P=9