Hum, let's see?  You don't, unless of course, that person is aware and is completely honest with you.  the possibility of that can be rather slim.  80% of people with an STD or STI don't know they have it.  And out of the ones who do know 80% choose not to thell the person they are dating and will flat out lie if asked.  You could of course, check them yourself, right?  Not quite, most people with STD's or STI's don't have any symptoms.  This is one reason people don't know they have any to begin with.  You could have them get tested for everything, right?  Not exactly, while many STD's can easitly be tested for, others cannot.  And some require repeat testing after a period of time to completely rule out.  The only absolute way of knowing is being in a long committed relationship with someone who has never had any form of sex with anyone.