put the rolling stock on it and see if the wheels sit on the track. Most track that is narrow gauge is names NG3 or NG2 etc..... and most rolling stock has the same naming scheme.
Yes - trains in Trainz are not gauge specific. An 18 inch gauge loco will run happy as a clam on broad gauge (7 ft 1/4 in) track. Don't ask me to tell you what it looks like tho, lol.