In order to get a full screening for all STIs, we need to ask for one expressly
Want to ditch the barriers? IF after around six months, partners have ALL a) remained in an exclusive sexual relationship with each other and b) EACH have had a new, full STI testing with negative results at that point OR got a positive result for an STI that was treated fully, then most STI risks have likely been radically reduced and will remain reduced so long as partners stay in an exclusive relationship, so it’s considered fairly safe to stop using barriers if you want. (There is still no approved test for HPV for people with penises, so HPV risks may still exist with partners with penises who have otherwise negative results from STI testing.) This isn’t just about two-person monogamous relationships: with closed poly relationships — where three partners, for instance, are only with each other sexually and no one else — where all partners have followed the protocols above, people are likely just as safe as closed relationships-of-two where those protocols have been followed.
IF you’re in an open relationship or any relationship where everyone isn’t keeping sex to a closed, communicative safer sex triad or group, or you’re not 100% sure if a partner is really being monogamous or really got tested, it’s ideal to stick with barrier use no matter how long you’re together.
Step Two: Testing & Treatment
The only way any of us can have an accurate idea about what our infection status is — or what the STI status of our partners may be — is through STI testing. Most people who have STIs don’t have obvious symptoms so that 
they can know and more people with STIs don’t know they have one than those who know they do; most folks with most STIs feel and look just the same as folks without. Some people think they’ve been tested if they have donated blood, had a pregnancy test or got a regular checkup, but a full STI screening isn’t usually part of any of those things.
There are a lot of different places to get tested: general or sexual health clinics, private OB/GYNs or urologists, your family doctor, urgent care or hospitals
Unless you or a partner have been to a clinic for STI testing, and asked a healthcare provider for a full screening for STIs, it’s highly unlikely you’ve had STI tests or complete testing. A whole lot of people who are or have been sexually active also haven’t ever been tested, and some folks who got tested for a couple STIs once think that means they have no STIs or can’t get them ever after, but that’s not true.
To have the best idea of what we’ve got and what we don’t, we want to get tested around once every year as a habit; more often if we’re switching partners more frequently, or we and/or a partner are not being or have not been sexually exclusive with only each other. In a new relationship, if someone has had other partners before don’t figure anyone starts off automatically STI-free because it’s a new relationship: we need at least a few months to pass before getting tested again to be sure. It’s always key that all partners get tested, not just one: a partner’s status is not always an accurate reflection of our own, and our tests won’t always show us what infections our partners may or may not have.
If you live somewhere with national healthcare, some STI tests are usually covered. If you live somewhere without socialized medicine, insurance may cover your testing. If you do not have insurance, many states or areas public health departments can provide STI testing at low or no cost, whether you are or are not enrolled in a public health program. What a given test involves varies, so take a look here if you want to know which test is for what. You can always ask for a full screening or can talk to your healthcare provider about your sex life and ask what tests they recommend for you.