On a Remington 700? If the chamber size isn't known, or the receiver isn't barreled, measure the distance between the receiver screws. If 6.5 inches, it's a short action. Likewise, if the screw-to-screw distance is 7.35 inches, it's a long action. On Savage & Winchester...
Winchester short action - 7.03"
Savage short action - 4.28"
Winchester long action - 7.57"
Savage long action - 5.06"
Shorter rounds like .223/5.56mm, 308 Win & 243 are short action. .30-06, .270, 300 Win Mag & longer rounds go into long actions.