Put the ramrod down the barrel and mark where it stops and then lay it next to the barrel with the mark even with the muzzle. If that ramrod reaches all the way to the line where the breech plug starts, then the gun is empty. If it stops a few inches before the breech plug starts, you have a loaded gun.
Also, a ramrod dropped down the bore of an empty gun makes sort of a "tink" sound as it bounces off the back of the breech. If it hits a lead ball, it doesn't make much of a sound and it doesn't bounce much.