Look for integrity of the frame and make sure all nuts and bolts are tight. Some of these have slides where the mains slide forward and backwards. Make sure the slides don't hang up and there is no wear at this point. Now we get back to where most harnesses are similar. I start at the shoulder straps and work my way to the toe.
Most shoulder straps are adjustable.
Make sure there is no wear at this point. Also check the adjusting buckle for signs or wear. Moving down to where you get into your harness. If it has zippers check to make sure that they work properly and the stitching holding the zipper to the harness is good shape. Most harnesses with zippers here will also have one or two safety clips.