What can one really determine, online or otherwise, to find out if a person is being truthful about themselves, their history and credentials?

“Background checks” are sometimes vague and limited in scope; limited to felonies and crimes of moral turpitude, Social Security records of employment, etc etc. But not why they left their employment, or if they did as well as they claim, or accomplished what they said, or even are who they say they are.

What, then, can a regular person determine about a person they intend to employ, date. or even meet or trust in some manner?

How can the average person determine if someone has the degrees they claim or even went to the schools they claim, or worked where they said they did?

How can someone know if someone has the same educational credentials, work experiences and accomplishments, work history, etc. How easy is it to fake or alter this information?

(Edit) I realize that there are websites that claim to find this information for you But can this be faked, altered? How can such said websites find the information they claim to have?