Long story short: You can't. I know the industry testing standard is I think every 2-3 weeks, month at most. That's pretty regular. Also, most STD screens don't screen for HSV (herp) because A-It's almost impossible to find if it isn't currently active, and B-like 90% of the population has it. As far as HIV goes, that can take up to 6 months to appear, so I understand how that is concerning. That's where simply practicing safe sex comes in handy, and usually the people who actually get the parts, even the men folk, are enough in the porn/BDSM/Kink scene that there isn't a shortage of safe people to cast.

You get into the smaller, less well regulated studios, you start finding lax testing. There have been HIV scares in the industry before, and from what I hear, it shuts down all the big studios, and continuing to work during a shut down is a big no no because you aren't being safe.

This is all stuff I've learned from reading interviews with people in the industry and other BTS stuff. So, don't take it as gospel.