Not an expert by any means here, but a few observations I have made while hunting and trapping. Fox tend to leave bits of bone and hide outside their holes as well as some scat. Their holes tend to be oval shaped from top to bottom as well. Skunks and coons tend to show up in oval shaped holes that are wider than tall since that is how they are shaped. Groundhogs will dig this way too when they are larger from what I've noticed, but the smaller ones seem to dig rounder holes. Skunks and coons tend to have trash near the mouth of the hole like paper and trash bags and leave a decent bit of scat around. I don't notice much groundhog scat at the den mouths where I kill them and they are more likely to have a mounded entrance or no visible entrance(not sure what they do with the dirt
). They usually have more of a smooth belly slide going in and out of their normal entry angle to the hole and will pack the bottom of the holes more when the angle of the hole is flatter.
On an interesting side note, in a single month of trapping, I have caught a red fox, skunks, possum, coons, groundhog, and feral cats at the mouth of the same den in the month of January. There is a pile of droppings outside this hole in a rock quarry face that is 6" deep. I have nearly stepped on several red fox that were coming out of groundhog holes as I was stepping over them too. Some of these holes I had just seen groundhogs in within a few weeks before. This seems more common in spring for me.
Your best bet is to get a track or some hair, or better yet, trailcam footage. You can sift dirt around the hole in hopes of getting a fresh track or hope for some fresh mud and get a good print. While groundhog hunting, sometimes I'll purposly stomp around a hole or ride over it with the ATV then sit back and watch it to see what pops out. You'd be surprised how many times a chuck has popped out within 30min to see what the ruckus was. They do it all the time with tractors.