I don't know what a DCRA .303 rifle is.
One used in DCRA/PRA competition?
At a minimum, there could be a precision target rear sight, and a center swivel. This for SR(b) shooting. For SR(a), neither of these add-ons would be present.
Bedding might be improved. Barrel might be ball burnished. There could be marks indicating that a particular business did enhancements. Forend could be a custom, in issue configuration. A SMLE might have another barrel band added behind the nosecap, and could have all sorts of weird and wonderful forend bedding.
Often PH, AJP, Central, Swenson, etc. sights get added to rifles, even if the rifle has never been used in competition.
Provenance may be the best indicator. And if you can find out who used the rifle, the records for their shooting will have survived.