Interesting ways to tell who you are dealing with

In business, knowing what kind of person (supplier, customer, partner) you are dealing with could save you from thousands or even millions in financial losses and heartache.

My uncle, a Fortune 1000 executive, told me the best way to read a person is to gamble with them. Whenever he meets a new customer, supplier or new colleague, he would invite them for a game of cards. He said scumbags and cheats cannot pretend to be honest when a game of chance + some of their pocket money is involved. It's in a cheater's nature to cheat, and a petty person's nature to be petty, and like a scorpion on the back of a frog, they will reveal their true character.

My aunt, a owner of an export company, told me when she meets a new business contact, she would take them out for dinner and when the bill comes, she would sometimes say she forgot her wallet and she would gauge the reaction of the other person's face. Of course, she ultimately won't let her guest pay as she would pay with a credit card that she claimed to have found in her purse. I tried this once myself and the other person's face turned dark, and that guy later turned out to be a complete dickhead of a person.

Are there are tips and tricks that fellow redditors would like to share?