Online autism tests
There are a few multiple-choice autism tests you can take online, to get a rough feel for whether you might be autistic or not. Some of the main ones are:
In an experiment, 80% of the autistic people who took the AQ test scored 32/50 or above, whereas only 2% of the allistic people scored that high. Bear in mind, however, that this test is imperfect, and biased towards confirming autistic people who happen to be straight, white, cis men.
As you might expect, the cut-down version of the test, the AQ-10, shows a similar correlation, only much coarser. It also helps to show that self-assessment is valid: if you think you're very likely to be autistic, then you're probably right.
It's important to bear in mind that none of these tests are as accurate as getting a professional diagnosis from someone neurodiversity-affirming (as in, someone who actually understands what being autistic subjectively feels like, not just what it objectively looks like to outsiders, so they much better understand why we do things the way we do), who you can talk to, and who people who knew you as a child can preferably talk to as well.
Still, these tests are better than nothing, and can be a good first step.
Really, though, your best bet is to spend a while talking to people in the autistic community and seeing if you relate more to us or to allistic people. You don't need to be officially diagnosed to be welcome, or to start making various aspects of your life more manageable.
Simpler accommodations include things like wearing earplugs, and leaving crowded rooms when they get overwhelming. Even if it turns out those somehow help you for reasons unrelated to autism, that's still a good thing that they're helping you, whatever the reason.
If you need to do things like fidget or bite your nails, then it's OK that you need to do them, regardless of the reason. You don't need anyone else's permission to be comfortable, and you don't owe anyone a reason or explanation either.
And if you think the questions on these tests are too ambiguous to answer... that's probably a sign.