Normally when escorts say they are clean it's true, as suffering std would mean loosing money.
Oral sex exposes you to several stds though.
-Syphilis, you can test after 6 weeks, and if you get it negative, it would be interesting to re-test 3 months after the exposure, as there are people that take longer to show a positive result.
-Gonorrhea, you can test 3 weeks after the exposure.
-Herpes, you can test 3 months after the exposure.
But as I said, the risk is really low.
All the best.