Harder than you'd think (and wish): if this was signaling "Columbus" then you'd check the fork-steerer base for "rifling", but with Reynolds (and the oddball, for a frame, "Campagnolo" decals) nothing can be taken at face value. IF you were to strip all the paint (not my reco) you might luck out to find some very small and faint stamps on the tubing which could say "reynolds 531 db" but often those stampings are cut off or buffed out. Weight compared with a known 531 PRO frame of the same style and size might be the best (but still not 100% conclusive) bet.

Just one more trivia bit: according to this chart of Reynolds decals the forkblade shield IS correct for a 531 PRO red label frame, but it's mentioned the fork blades are "wide oval" so might be something you could measure (at the blade top near the crown) and compare the dimensions, at least if they are "wide-oval" dims then the fork might be legit.

http://www.equusbicycle.com/bike/rey...-transfers.jpg