There are certain ways to tell. If you are getting bites where it seems like they are on for a second or two and you don’t catch it they are probably moving your bait away from the nest. If you get bit in exactly the same spot they could be spawning. As Champ said. If they are in similar depth in a given pocket they could be spawning. Some bass eyes turn more red during spawn too. Then of course you have the bloody or worn down, rough looking tail. I’m sure there’s more ways to tell, but those are a few.