The proper way to check chainline is from the side of the seat tube to the tip of a tooth on the middle ring, then add half the ST diameter. The normal chainline for a Shimano road triple is 45mm and an MTB model is 50mm.

If there is a spacer provided, it goes on the right side, to make room for the third chainring.

Some crankarms may be offset by a few millimeters. A better way to check that is to rotate the arms parallel to the downtube and check the distance from the downtube for each one.