You need to start with making sure the sub frame is square. Check it to the body and check it within it's self.
Here are a couple of examples. A to B should be the same as C to D right?
8 to 5 should be the same as 7 to 6 right?
Find "control points" that being a spot on both sides that are identical, those rad support holes in the frame are perfect. The back of the subframe isn't so easy with it in there. But you could even go from the center hole on the crossmember out to the frame horns to a hole or to a corner to see if the frame is bent. Go to a number of different spots to verify you are getting the same amount off or on to be SURE you are getting a good reading. Go to the body somewhere to be sure the frame is straight on the body.