You definitely can't tell whether someone has an STI by looking at or talking with them. In fact, most girls and about half of guys DON'T get any symptoms so they don't even know that they have an STI!
Symptoms for girls can include burning or pain when passing urine and unusual discharge from the vagina. Guys may experience whitish or yellow discharge from the penis and burning or pain when passing urine.
Whether you have symptoms or not, STIs are causing you damage on the inside. Depending on which STI you have, it can lead to you or your partner becoming infertile (not able to have a baby in the future).
The only way to protect yourself against most STIs and unplanned pregnancy is by using condoms and water-based lubricant. And the only way to know for sure that you have or haven't got an STI is by having an STI test which is usually just a simple urine test.