If your well is the type of wells I've seen (steel casing, say 4" to 8" diameter), I'm guessing you'll have some problems with acoustic detection because of the scattering that will occur with the stuff going to the pump. But the only way to know is to try. I've pulled my pump out and I've banged on the casing and listened to the reflections, so the basic idea is sound (no pun), but the devil will be in the details.
Again, I think one of the best things you can do is to call some local pump places and see if there are any commercial tools available.
I like the idea of measuring the water pressure (nice idea, JimB!), as it stands the best chance of being simple and giving meaningful measurements with the least amount of work. I acknowledge Ron's concern about the pressure measurement when the pump is running, but I think the effect will either be negligible or could be corrected for. A letter to a pump company's engineering team might get some results.
In fact, next time I pull my pump, I'm going to put a pressure sensor on the outside of the pipe just above the pump and try this method.
BTW, I've pulled a few pumps up and have learned how to do it manually -- it doesn't take sophisticated equipment as long as the pump's not too deep or the thing's too heavy. It's something technical folks can do with a little care. If there's any interest, I'll write up a blurb on the things I've learned about it. A nice thing about doing your own work is you know the thing was done correctly with the right parts.