Seriously, Cougar. If there was an easy way to tell, STD's (VD in my day) would have been a thing of the past a long time ago. Good luck with that routine.
You can get herpes when there are no signs of it.
You can get gonorrhea when there are no symptoms.
You can get aids from a person who has it and doesn't even know himself/herself.
Genital warts, Chlamydia and syphillus can all be caught without the carrier having any telltale signs that only a clinical test would reveal.
If you don't wear a condom, you just have to take your chances. Not recommended but your call.