I cannot hear crickets with my right ear.
My wife hears them to the point they drive her bats. I ignore them till she bugs me enough to go on a seek and destroy mission. Of course she has to be my fire control coordinator as I can't tell where the little bugs are coming from.
When it comes to noises in the trucks ..... I tell her to wait till it gets loud enough for me to hear it, then I'll fix it. That's about all I can do.
"Most" of my machines have the top tension set between 2 ½ and 3 ½, using #50 thread. My wife has pretty much taught me how to "feel" the thread to know weather the tension is right or not. So when I have a machine that's close at 3 ± I'll tweak the bobbin adjustment to bring them together.
As for checking the bobbin, I've played with them enough that I normally can feel if they are right just by gently pulling on the thread after the needle has pulled it up through the needle plate.
A very "scientific" method. LOL
I do the same type of things with the round and long bobbins. If there is no dirt, or mechanical problems they all respond pretty much the same.
Joe