A string line across the top helps with the CL and normally for height I get a good centered look up the back of the tailpipe and see if it is high/low/or just right..
Sight seems to work best for me too. I actually tried to make an alignment jig once and when I checked it by sight I could it was off slightly and needed to make a minor adjustment. It's amazing how small a misalignment you can detect visually..