In my limited experience, you could tell if a fireman knew his business or not by getting on an engine that was just brought to the roundhouse after a run and looking into the firebox.....A fire that was fed carefully and properly on the road would have burned down to not much other than ashes, which could then be easily shaken into the ashpans and dumped. Someone who fired incorrectly, heavy in places and too light in others, or over fired when the engine wasn't being worked hard enough, or raked green coal underneath the fire after overfiring would leave an engine with a bunch of clinkers in there, which you had to bust up and fish out thru the firedoor before shaking the fire down and rebuilding it.
We were taught (if paying attention) to turn the scoop over in the firedoor and look at the fire to see if you had light or heavy places you needed to fix. Then if you fired correctly with a proper pattern, you could keep the fire just about right and avoid having to deal with a bunch of clinkers when you stopped for water, etc. Took a bit of practice to learn, but I never had much trouble with clinkers or having to really clean a fire on the road very often. ....Durango end usually had good coal from the mines to the west of there.