Dr. Tony Attwood, one of more well-known experts in autism,...admits that he missed his son's diagnosis for many years. It wasn't until he started looking at old home movies of his son that he picked up on the behaviors he overlooked.
Just because you don't have a known family history of autism means nothing. I wasn't officially diagnosed until 52,...in part,...because the ASD-1 variants were generally overlooked until the 1990s. Theoretically, you and I could have had a several family members over the years with the so-called "milder" variants of autism.
There are some on-line tests that, depending upon your scores, will suggest autism,...the AQ score, the RAADS score, etc. and there are other tests on the Cambridge University Autism Research site. As others have found out, "If it has feathers like a duck, sounds like a duck, has webbed feet like a duck, has a bill like a duck,...it might be a duck." Self-diagnosis is something that is common, for several reasons. However, if you are in a country where someone with an autism diagnosis may legally qualify for "special considerations and/or accommodations" with regards to the medical system, legal system, and/or employment protection,...and you wish to take advantage of those,...you need that official diagnosis prior.
At any rate, welcome to the forum.