I have never personally seen a hot coal in a barrel, but it is easy to see smoke and hot gasses coming out the muzzle and the vent. When I see that after a shot, I blow down the barrel. If the gun did not go off, you would not see the smoke.
You should know your gun well enough to judge where the ramrod should be, loaded, unloaded, not seated, dry ball, and double charged. Marks make it even easier.