FWIW, I use the following method.
1) Choose one (and only one) of your units in combat.
2) Note any of your opponent's units it touches, they are in the same combat.
3) Note any of your units they touch, they are in the same combat.
4) Note any of your opponent's units they touch, they are in the same combat.
5) Repeat steps 3 and 4 until no new units are added.
I then move on to another one of my units that's not in the above combat and do the same.