Y'all, I finally noticed something about the Engines in-game.
When I opened up the music album that Hans created for us, I noticed that for each engine there are three kinds of music, one for High fog (which we hear most of the time by default) one for Medium fog (sometimes heard) and one for Low fog (only heard when an engine is full)
The fog level is indicated by the music!!
For the NML's Tower Engine, you can hear the different music in the main NML room. Only if you hear the Low fog music can you enter the Junkyard!
For the Beach engine, you can hear the different music in the Beach engine room. Again, if you can hear the Low fog music, you can enter the Greenshade Caverns!
For the Slurry Engine, you can hear the different music in the Slurry engine room. If you can hear the Low fog music, the Forgotten Labs are open!
I used to get frustrated by not knowing if the Engines were full or not, and having to trek to the nearest locked room door to test the Engines aether tank level.