Secrets to evaluating a purchase

When, after months of searching, you find an owner who may be interested in selling, what do you look for (beyond all the basics) that tells you if they will be a good purchase?

For example: I have a good friend who is a dentist and has purchased over 20 practices. He would go the back room where the owner kept the billing files and spot check through many of the thousands of records. He was thrilled if he found a lot of old paper records, inconsistent visits and many procedures outsourced. All opportunities to buy at a discount and grow the business.

I have another friend that buys apartment complexes. He has one of his people watch over the place and do parking lot counts to see how many cars are in the lot at different points during the day and night. His logic is that if it's full during the day, they are either working from home, night shift workers or unemployed. Based on his experience, they are the people that have more trouble paying when the monthly rent comes due. When it's empty during the day and full at night, that's his best scenario. Those are the people with steady day jobs and are moving up in the business.

I'm interested to hear what hypothesis you draw when you evaluate different opportunities and how they have played out after the purchase.