Nitrocellulose lacquer can turn yellowish over time. However a lot of companies also tint their lacquer, so that's not really a good way to tell. The wear patterns with nitro, acrylic, and epoxy lacquer are going to be approximately the same. I've got a 1994 King trombone, which is not nitro, and a 2000 Bach 42T, which is, and they both have about the same about of wear in the same places. I have not really noticed much of a difference of durability to mechanical wear.
I think really the only way to tell is to know what make of horn it is, and when it it was made. I have two Kanstul trombones, and I have no idea what type of finish they used.
Sterling / Perantucci 1065HGS Euphonium, 1952 B&H Imperial Eb Tuba, and a bunch of trombones.