How do you ask someone if they have an STD?!

I would just ask outright, however just because they say they are clean does not mean it is Ok to not wear a condom. A lot of infected people have no idea that they had an STD. Chlamydia has no symptoms a lot of the time, neither does herpes unless they are in the middle of a break out and we all know someone can be HIV positive for years before getting sick.
Wether your relationship is gay or straight it is never worth the risk to go bare back until you have had tests done. When I met my husband we both agreed to get tested before we stopped using them.

There's not much to do about it other than to just ask. One way to bring it up might be to volunteer your own testing status; when you were lasted tested, for what, and what the results were. Sharing that information yourself makes it easier for the other person to share their information, and makes it clear that you're serious about protecting your health. Asking about testing status gives you more information than just asking if someone has an STD, too, since someone who doesn't know of anything but hasn't been tested in years is a higher risk than someone who tested negative within the last few months.

Of course, this strategy does require you to go to a clinic and get yourself tested for common STDs. But you really ought to be doing that anyway. Talk to your doctor or other health care provider about how often you should go in for STD screenings based on your number of partners; once a year is a pretty common schedule.

You can also protect yourself by using condoms every time you have sex. They can be annoying, but they substantially lower the risk of catching most STDs.

before you do anything with them just be like "your clean right, and dont lie." ive done it i say the same thing every time. "do you have any std's? before i hookup with them. and they always just give a look.and i go "dont lie to me, just be straightup and tell me if you do or not. its not gonna matter just tell me and if i find out you lied im gonna kick your ***." every timeee. its worked. one guy told me he had chlamydia and he was honest about it thankgod

my sex life

just be up front and honest, its kinda like bringing up protection during sex (no matter what its going to be awkward but its gotta be done) . . . and congrats for you for thinking ahead ?

Don't ask don't tell.

"when were you last checked for std/sti's? are you clean?" that's usually how i word it. good luck! and be safe!