Being gay does not affect the chances of you getting HIV.
I second what Teak just said... your risk is very low, but all people who are sexually active outside of long-term, monogamous relationships should be tested for STDs and HIV periodically.
On another note, I highly encourage you to inquire about HIV status with the partners you have sex with before having sex with them. If you are concerned, you can speak to your partner (maybe they have been tested). It's up to you whether you would like to test or not. For the future, you may also choose to use flavoured condoms for oral sex (as this will virtually eliminate any chances for HIV). It's a personal decision to make though, you may choose to decide that the risk of HIV from oral sex is low enough for you to continue your current lifestyle.
If you have good hygiene the risk is zero. If your hygiene not good then it is a very low risk. You're correct you cannot look at someone and tell if they have HIV.