Defence Industries (DI Z) tracers are marked on the bullet point with paint.
Properly speaking, calibre .303 tracer should be marked on the BASE of the casing. Stuff from the period of the Great War will be marked VIIG; this was changed in 1926 or 1927 and production from then onward should be marked GII, GIII, something like that, adding a Z if the propellant is NC powder, leaving no propellant marking if Cordite is used. Also, primer lacquer used on Tracers was different colour.
It's not unusual to come across Tracers, but watch out for Incendiaries, marked with a B code (from "Buckingham" originally), which just might be grabbed and you could be charged, depending on who the LEO in your area might be. It's all so silly!
Tracers are just plain fun, but should be used with discretion and the barrel MUST be cleaned IMMEDIATELY following use of even a single Tracer. They leave corrosive crud in the bore..... and new barrels are getting just plain difficult to find, not to mention expensive!