I built a 12' x 25' enclosed porch about 18 years ago off the back of my garage in Michigan. The porch is supported by 10 concrete piers , each 42" deep in the ground. About 5 years ago I noticed that the French doors on the south and west sides have a larger top gap on the latch side than the hinge side and drag when closing. I suspect that the porch is settling, though when I crawled underneath, there is no visible deflection or rot in any of the load bearing surfaces. My question is, how can I best determine where the sagging has occurred? Is there something I should be looking for specifically - a typical failure mode? Thanks in advance for any suggestions.