I'm sure Boco will tell us, but I'm assuming by the tracks. I know I had a new pack move into an area this year. There was a bunch of three in there last year, all big adult wolves, I took out an old male, leaving a female and a male (could tell by them peeing in the snow, for completely positive sex ID). There was a big bunch with some small tracks in it this fall, that I assumed was that pair with a litter of pups. But I caught six out of them last week, an old, almost toothless female, an older male, two pups and two two year old males. So that is not the pair that was in there with a litter of pups. I think I cleaned that pack, and a couple days later seen sign of a pair in there again. Probably the old pair who has been living there.
Once you find the locations where two wolf packs mark territory at a boundary you will see if you monitor the location over a few years that when one pack is thinned out the adjacent pack will know from the regular scent marking that the pack is weakened and will start to make incursions more frequently or in some cases take over new territory if their numbers are left to increase.
Once the individual wolves in a pack come together after freeze up to hunt moose,you can go out near the boundary location that you have identified on a cold clear night and do some howls or even moose calls.Often you will get wolves howling right away,then short time after you may hear another pack howling from a totally different area
When I stayed at the camp on my more remote Mattagami line in the late fall before freeze up I would hear wolves howling west side of the river,then sometimes another pack would start singing over on the east side of the river.
Wolves communicate a lot.The times the hair really stands up on my neck is when you hear them whistle barking to each other in the bush very close and on several sides of where you are.
The first time I heard that was one fall I was checking beaver traps and seen fresh wolf tracks where a wolf had pulled the trap and beaver up on the bank,but had not eaten it,I was sure from the sign I had chased it off when it heard the bike coming on the trail
As i reset the trap and took the beaver I could hear 3 wolves whistle barking just out of sight in the brush on 3 different sides of me.
Had similar happen a few times since,and Ovila Sylvain a big number local wolf trapper had told me about that before I had experienced it so I knew what it was.