There's a table, and a medical grade paper towel here, what do you think is gonna happen?

When a virus enters your body, the immune system creates antibodies to fight back, Crash Course explains here. Antibodies are matched to a specific virus' design. What I mean is that there are different antibodies produced for different types of viruses or pathogens. In the case of HIV, the Human Immunodeficiency Virus, the immune system responds with HIV antibodies. They don't fend off this intruder but they do try. There are a variety of testing options to find out whether or not HIV is in your body; most of the ones you are going to come across are antibody tests, meaning they are going to look for antibodies that would be fighting off HIV and not the virus itself. This test is cheaper, faster, and easier to administer. There are even some pharmacies that sell home kits so you can do it yourself.

You are at risk and need a test if...

human blood, seminal fluid, breast milk, or vaginal fluid from another human being has come in contact with your mucous membranes. Mucous membranes include: eyes, ears, nostrils, mouth, urethral opening, vulva, vagina, anus, rectum, and any other slimy part where your internal organs meet the outside world.

If you aren't at risk for HIV then hold off getting tested. The only reason why I see doing this preemptive test is for you to go in when you're not anxious about the results so that you know what is going to happen when you are anxious about the results.
I'll save you the trial appointment. Here's what it looks like. Medical grade towel, little blue tray, test.

Before anything starts, the person testing you is going to explain what's about to happen before it happens. This starts with an explanation of the window period. The time between exposure to the virus and when there are detectable antibodies that are trying to fight it off.

Let's use this milk as an example. It's like your immune system, uninfected. Enter Cocoa-dusted rice puffs. The test is not looking for the cereal. It's looking to see whether or not the immune system is responding. Whether or not it's turning chocolate-y. This might not happen right away. See. White, white, white.

Same with the window period for actual HIV. Now it's brown. This could be anywhere within three months. In rare cases up to six months. Because there is no obvious or clear way to know where you fall in this window period range, a negative or non-reactive test result really only applies to three months prior to that test. This is why I recommend HIV screening at least once a year, if you put yourself at risk. Like by having sex. 

The person giving you this test wants to make sure this is comprehended because if you have had any risk behaviors in the last three months then the test could be inaccurate. And maybe you should hold off until we can get an accurate read. Today we are not holding off. 

This is an OraQuick advanced HIV 1 and 2 antibody test. It's looking for antibodies for two different strains of HIV. It can used with a blood sample or an oral sample. Not saliva. And it takes only twenty minutes to process. There are two pouches in this test. One side has a developing fluid which activates the test, this little vial here, where I'm going to pop the cap and set in in the little blue tray. The other side has the actual test itself.

On the end, is what is called a flat-pad, it's kind of like a communion wafer. To do an oral swab, I do one smooth stroke across my top gums, turn it over, and do one smooth stroke across my bottom gums. None of this. 

When I've collected the sample from my gum tissue, I put the test in the vial and start the timer at twenty minutes. During this this time the testing counselor would usually chat you up. Find out what you know about HIV. How you think you contracted it. This person has probably been exposed to a full range of sexual behaviors. Feel free to talk about anal sex, glory holes, orgies, blood play, whatever you need to be thorough. The point is for you to know your status and to stop the virus from spreading. Oop, timer goes off and the test reads. 

Wait, wait, first let me explain what you're looking for. This is a blown up version of the testing indicator. There is a 'C' for control and 'T' here for test, the results. Both of these have a bracket and we are looking for lines to fall within those brackets. If there is a horizontal line within the 'C' bracket it means the test is working this is what we want. If there isn't one within the 'T' bracket the test can be interpreted as non-reactive, HIV negative. If there is one it's reactive, HIV positive. Here's what the test looks like when it doesn't detect HIV antibodies. There are variations in how testing will happen, some clinics are going to want to test with finger sticks, blood they are going to take longer to process the results, counsellors may give out condoms or clean works or maybe there are testing operations in the back of adult bookstores or in vans. The goal of them all is to get people tested and stop the spread of the virus, they don't want anyone to feel ashamed or scared. I hope in the comments below there can be helpful conversations about additional resources and encouragement for people to get tested. Maybe somebody can clear everybody in on glory-holes. You can click on my face and hear me answer the most frequently asked questions I get on HIV. This is not something to be afraid of! See?