The best way is to take a SPL and set a level for the crossover region(like 80hz) Set the subs phase for maximum output at this level with the main speakers and it should be in closest phase. However, room anomalies can change this situation somewhat. I have found it is pretty easy to tell when they are in phase. Place your head between the mains and subs and listen. It shouldn't sound like there is a suckout.
How do you use subwoofer phase control ?
Recently obtained 1986 era Duntech Thor powered large bench-sized subwoofers with outboard crossover box. No instructions about dialing in the phase somewhere between 0 to 180 degrees. What does one do to choose an optimum setting? Is one sub adjusted separately, and then later the second if allocating one sub per channel? May simply be using one of the subs equidistant between stereo Duntech wall mounted speakers as room size dictates due to massive "coffin" appearance of each sub. These subs specifically designed to complement these wall mounted speakers. Quite a contrast from the modern micro-sized subwoofers such as a Carver.