If I'm reading it correctly the 1997 MRI standard for commercial shelving says to test to 1.5 times the design load, measure the deflection, leave it under load for 15 minutes, and measure the deflection again. If it increases by more than 5% in 15 minutes, or stays deflected more than 15% of the final deflection after the load is removed, it fails the test.

http://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1000&context=ccfss-rmi

There's a newer standard but it doesn't seem to be available online.

If your mirror weighs 112 pounds you would want to test to 112*1.4=168.

Personally I would test with milk jugs or something (figure a gallon jug of water is 8 pounds, a 5-gallon bucket is 40 pounds) rather than standing on it--if it breaks, water on the floor is a lot easier to deal with than broken legs.