Get screened for whatever you think you've got. Think you've got ADHD? Go get an ADHD screening. Autism? It's not easy to find a psych who does adult Autism screenings, but they're out there. OCD? You get the idea.
Regardless of whether the conclusion is "yes you have x" or "no you don't have x" the diagnosis will be accompanied by a detailed analysis of your psychological condition. Whether or not you are diagnosed, that analysis will cover the issues that led you to believe you may have that condition.
Mental health, like physical health, requires action on your part. If the only thing you're seeking from a diagnosis is accommodations from others, then yeah, you're probably going to be disappointed.
I think you mean the full diagnostic process? Screening is just the first step in (what should be) a long process to decide whether the full workup makes sense. Screening takes on the order of minutes to an hour and it doesn't come with a diagnosis, the actual diagnostic process should take many hours and several appointments.
Regardless of whether the conclusion is "yes you have x" or "no you don't have x" the diagnosis will be accompanied by a detailed analysis of your psychological condition. Whether or not you are diagnosed, that analysis will cover the issues that led you to believe you may have that condition.