Here's a tip for you

"I went into a French restaurant and asked the waiter, 'Have you got frog's legs?' He said, 'Yes,' so I said, 'Well hop into the kitchen and get me a cheese sandwich.'"  Tommy Cooper

It's long been acknowledged that a good way to determine character in someone is to golf with them.  Over the course of 4+ hours out on the course and probably drinks afterwards you get to spend half the day with someone under conditions that show what's under their skin.  Will Rogers once said, "only the income tax has made more liars out of people than golf."  A round of golf with someone allows you to see their temperament, honesty, competitiveness and communication skills.  It also allows you to find out if they have a mean streak, like to gain unfair advantage and how they react to adversity.  Believe me, two lost balls and a three putt on one hole will bring that out.

The problem is, not everyone plays golf.  Plus as previously mentioned it takes up the better part of a day.  So it's not the most practical way of scoping out a person's true personality.

There's other ways though.

Tipping, along with how someone treats service people, is another great indicator.  Tipping by itself can be deceiving.  Some people have a set rule that they never vary from and as long as it's not unreasonable you can't take them as a bad person because of it.  There are also extreme circumstances where reducing or withholding a tip becomes necessary.

I tend to pay more attention to how others treat service people.  People that treat wait staff, valets, receptionists, clerks, etc. with an air of indifference or even worse distain on a consistent basis always bother me.  I've found some of the best people to work with and for are those who are overly friendly and accommodating to service people.  They acknowledge them, chat with them, thank them and more often than not over tip them.  I have found these types to be the best of do business with because they tend - typically, not always - to be more trustworthy.

The one trait that I've observed firsthand a couple of times are successful people who upon realizing they've accidentally been undercharged or given too much change back happily put it in their pocket and once out of range crow about it to those they're with.

On both occasions I put that person on my "watch carefully" list and with both I ultimately found I could not trust them in a business situation.

Regardless of the method it's worthwhile to watch people in normal, almost innocent, situations to see how they are.  If something strikes you as "off", pay attention.