Later steering gear has 7/8 diameter thread which requires a 1&5/16 hex nut. The earlier steering gear has the same 3/4 diameter thread as the manual gear.
Anyway... the hex nut is the quickest identifier.
Not to sound rude, but this isn't right. Both large and small sector shafts (output shafts) have the same 7/8 diameter thread and use the same nut. To tell the difference, you should measure the shaft just above the Pitman arm with calipers. Small sector is 1.125", large is 1.250".