The only way to know if anyone is who they claim to be is by talking with them, spending time with them, and observing them. Before you can "trust" someone they have to (demonstrate) honesty and integrity. You have to be willing to ask some tough questions when the time is appropriate. Only after getting their responses are you in a position to decide whether to believe them or not.
Trust like respect has to be (earned)!
If you find yourself becoming serious about someone you can always pay for a background search online such as http://www.pipl.com/ (free), http://www.intelius.com (FEE) , or http://www.peoplefinders.com (FEE) and http://www.zabasearch.com (free)
Most of these sites have options to view public records which will provide anything on marriages, divorces, bankruptsy, property liens, judgements, (background checks) will show any felonies, Aliases...etc
However the least expensive way is to (trust your inner guide).If you have some doubts don't ignore them!
"The purpose of the mind is to protect the heart."
If something doesn't "feel" right to you then it's probably not "right" for you.
Best of luck!