How do you know if you have MRSA in your blood stream but no symptoms?

You don't. Mrsa bloodstream infections almost always cause symptoms and frequently the patient is quite ill and toxic. In an immunocompetent patient it would be exceedingly rate for MRSA blood stream infection to not cause symptoms. Transient bacteremias do occur but they are brief and asymptomatic and usually do not cause disease but the bacteria can seed various organs and then cause symptomatic infection.
Blood culture. If you have bacteria in blood you will usually have symptoms such as fever, malaise and shaking chills and sometimes a drop in blood pressure if it is a severe and persistent infection, some people may have transient bacteremia( bacteria that are nutralized immediately by immune system after entering blood) , if you have no symptoms, you either have transient bacteremia or no bacteria at all.