How can you find AGL if the altimeter is set to MSL? Without doing the math, how do you know your true altitude AGL when your altimeter is set to MSL
I usually don't care a lot about my AGL altitude.

When I'm doing my flight planning on the ground, I do take into account terrain and obstacles, and pick appropriate MSL altitudes to be well clear of anything on the ground.  

But by the time I get in my plane, I'm thinking in terms of MSL (altimeter) and no longer about AGL.

That even applies to approaching my destination field.  If I know the field is at 1,500 ft. and I want to do a traffic pattern at 1,000ft. AGL, then I plan for my altimeter to read 2,500 (MSL).  And I think about if I'm at the proper MSL altitude (2,500), not what my AGL altitude is.