Thanks for your question.
Going down on a guy (or giving oral sex to a penis) is generally considered low-risk, but there are some STIs we see passed that way.
The STIs that can be passed by giving oral sex: Gonorrhea, Chlamydia, and Syphilis.
Gonorrhea and Chlamydia are passed through the ejaculate and pre-cum, and can make an infection at the back of your throat. These are tested by a throat swab.
Syphilis is passed through skin-to-skin contact (for example: penis shaft touching inside mouth). Syphilis is tested through a blood test.
As far as when to test… you want to wait at least 2 weeks after your gave the oral sex. You could also wait a month (4 weeks) and the test results would be even more accurate. Syphilis can take as long as 3 months to show-up after having sex, and you could re-test at that time as well if you wanted.
Please let us know if you need any additional information!