Is there an effective way to determine if someone has an std?

That's the problem: Unless the person has active & visible lesions you can't tell. A person may look completely healthy & normal but be harboring a dangerous & transmittable infection. This is why sexual activity is risky. Anytime one has sexual activity they should use protection for std's/ pregnancy. (unless of course they are in a committed, long-term monogamous relationship & have no std's & desire pregnancy.).