How to determine if i have an std or not?

Thorough H&P & labs!: You need a careful history and physical with appropriate lab tests to know. Drug use history very important as is number of sexual partners. After 5, virtually 100% according to cdc!

Medical eval/screen: Have you been sexually active without protection or engaged in other high risk behaviors? Female- vaginal discharge, bleeding w intercourse or btwn periods, nausea, fever, painful intercourse, lower abdominal or back pain. Male? Discharge, burning w urination, itching of urethra, pain/ swelling of testes. Any rectal pain, bleeding or d/c ? Skin lesions on genitalia or swollen lymph nodes? Etc.

Time for office eval: You can assume from exposure of symptoms that you could have a STI. However, sorting out which of the dozen or more possibilities would require a history,physical and targeted testing.