I assume, for you, most (or all) winter acquisitions have turned out well - meaning no unpleasant surprise in the Spring when things are easier to see.Has anyone had the opposite experience and found out in the Spring that there are large expenses for things like foundations, siding, or roofs that weren't detected because the property was inspected while snow and ice were present? 

All buildings should meet life safety codes and be in compliance with codes on smoke and fire detection, fire extinguishers, and carbon monoxide detectors.

But one of my hobbies is reading books about body language and deception detection.  

Even with info from a pay site you may need to develop you own detective abilities.

Plumber would snake it and it would work fine, couldn't detect a clog. 

This way if they do not meet the income requirement you can quickly detect that and give them back their app fee.Yes, I know in the pre-screening process most landlords go over the income requirement, but you would be surprised how many people have no idea how much money they take home per week/month.

An inspector should be able to detect signs of a problem.

I'd also suggest reading some of Mike Butler's books, he's a former police detective out of Louisville that did pretty much the exact same thing you want to do. 

You can sometimes detect a potential problem mortgagor in advance – while you’re checking out their credit and ability to pay – but not always.

You will have to develop a mechanism internally for BS Detection.