How can you find AGL if the 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.