What exactly do you mean by “pure of heart”?
Like @CWOTUS said, that term is for fairy tales not real life.
If you are asking how can you tell if someone is a decent person, the only way you can find that out, it to get to know them. You can’t know a person until you get to know them, by being around them, by watching how they treat other people, by figuring out if they do what they say they are going to do, if they’re hard working, if they are compassionate towards other people, especially more fragile people like the elderly and people with disabilities, and animals and like @LuckyGuy said, the waitstaff at restaurants and other customer service representatives, and people who are boring or not particularly intelligent. In my book, patience is a huge virtue.
When you meet someone, who is a potential mate for you, you should ask lots of questions at the beginning of a relationship. Don’t interrogate people, but ask them all sorts of questions like:
What are your parents like?
Do you get a long with you parents and siblings?
Do you like kids and animals?
Would you be willing to have dinner with my family?
What are your holiday family gatherings like?
What is your best friend like, can I meet him/her?
What is your opinion of gay rights and gay marriage?
Do you follow a particular religion? How do you regard people of other religions?
You don’t need to blurt these questions all in the course of one evening, but you should find out the answers to these questions, early on in a relationship.