For me it can be measured in the way that they talk to (and about) sales staff, waiters, their own family, friends and you.
When I met my OH, we lived in different countries, so it was difficult for me to see these kind of things. I recognised that he was a caring person by the things that he used to do for others, for example his mother. As in, taking her shopping, giving her a lift to the podiatrist, stopping by every Saturday morning then buying her fish and chips (her treat) and sitting with her so she wasn't eating alone (she was 83 at the time)
Unkind people don't do this kind of stuff.
Also, I used to work in a top class restaurant, and the people with "old" money were very appreciative to us staff, where as the "nouveau riche" as they were called there treated you like servants.
So if you pay attention to how people treat waiting staff, sales people etc. I think that gives you quite a good measure of how kind they really are and if they show and possess any empathy.
I heard somewhere that if you throw a glass of milk over someone, their reaction will tell you exactly what kind of person they are.
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