Traditionally the fish like gravel beds to spawn on. If you see large fish not being spooked off or kind of chasing each other around and returning to the same spot they are most likely on a bed. A good indicator would also be them fanning the gravel with their tail. These fish you should avoid.
A good way is to use your polarized and check the bottom. If you look closely you will notice the bed is much lighter in color than the surrounding area. This is a result of the "fanning" mentioned above. Even if there are no active fish on a bed, try to avoid them, as they can easily be destroyed by wading on top of them. Also, you can use a spawning bed to your advantage, by fishing lies close, and just downstream of the beds, as usually their are opportunistic fish attempting to eat the eggs, and bugs kicked up by the spawners.
