Does anyone know how to tell the difference between a TR and TRS by looking at the outside of the casing? I'm looking into buying a spare drive and the guy doesn't know for sure which model it is.

A TR is a longer length lower leg than a TRS
( they can run up to a 20" diameter prop)
Houseboats/work boats used them

It's like comparing a bravo 2 ( TR) to a bravo 1 ( TRS)

I know about the depth and prop thing, but is there a way to check bolt holes or something? I'm trying to tell the difference by looking at pics, and it's kind of tough.

The TR is 2:1 gear ratio and the TRS is 1.5:1. If the drive is off you can tell him to see how many times the input shaft has to turn to make the prop shaft turn one revolution. If you turn the input shaft 2 turns and the prop shaft only turns 1 revolution then it is a TR. If he turns the input shaft 1 1/2 turns to make the prop shaft turn 1 revolution then it's a TRS.

the lower unit on a tr is much bigger than trs, the top of the housing is the same

The TRS has a 1" propshaft.

The tr has a 1 1/4" propshaft