Yep, there's a blood test. There's no way to tell the difference without the
blood test or at least a culture. There are a lot of diseases, including
herpes, that symptoms like this could be. It could be something as simple as
pimples, or as bad as skin cancer. Most likely its pimples but you are going
to worry yourself sick until you get a test done, so just go get the test
and make yourself feel better.

The test will also allow you to have a "holier than thou" attitude twords
your less responsible friends! You can say things like:

"Sexual Health is everyone's responsibility! I had an STD test last month!
When was your last test? You need to know your own body to be safe! Condoms
don't protect you!"

etc... and then they will think about what a responsible and civically
minded person you are.