No. It’d be great if you could tell who had an STI (sexually transmitted infection, a.k.a. STD) and who didn’t, but the most common symptom of all STIs is no symptom at all. STIs are often asymptomatic or display such mild symptoms they go unnoticed and/or are mistaken for something else. For that reason, most people with an STI are unaware they have one.
When STIs are asymptomatic or exhibiting mild symptoms, they’re still transmittable to others—and there’s no guarantee the STI will remain asymptomatic in the newly infected individual. Recognizable symptom or not, STIs can wreak havoc internally long before we’re aware of their effects; untreated asymptomatic STIs can pose long-term health risks like liver damage (eg. Hepatitis) and infertility (eg. Chlamydia and gonorrhea).
To protect yourself against STIs, use condoms or internal condoms.