A lot of people with autism show no outward signs of it. One of your friends, coworkers etc. could be autistic and you’d never notice anything strange about them. Maybe they’re a bit shy or awkward in certain settings (in my case group conversations), but nothing major.
For those people autism isn’t necessarily a disability. As such, there would be no reason to treat them any differently than others even when you do find out they are on the spectrum. As a general rule of thumb, don’t assume anything about an autistic person. Find out what they’re like and respond appropriately, don’t assume they have the same strengths/struggles as your autistic nephew or an autistic celebrity or whatever