I bought a 629-4 Trail Boss a short while ago and pick it up at my FFL tomorrow. So just happens that I already researched this exact question......
Most obvious reason a "Trail Boss" is a "Trail Boss", is that it has "Trail Boss" engraved on the right side behind cylinder / by grip. But in general they were 3in with non-full barrel lug, round butt, and most (not all) were Magnaported. Most had lanyard loops at bottom of grip (not sure if its all, one on way to me doesn't but it may have been removed).
Here's the one I bought:
S&W 629-3 (1).jpg
S&W 629-3 (2).jpg
S&W 629-3 (3).jpg