One reason I don't tell people that I think they are autistic, is because that makes me and my relationship with them part of their self realisation.
Instead of simply dealing the question "am I autistic", now they have an additional tangle to unravel of "Zumbador is convinced I'm autistic, how will they react if I decide I'm not?"
Anyone with demand avoidance will be likely to dig in their heels and disagree with my opinion about their autism, simply because they don't feel safe being steered by me.
But I also don't think it's necessary to go out of your way to hide any hint that you think someone else might be autistic.
Depending on the person, you could tell someone that you wonder if they might be autistic. As long as you make it clear that you don't think you already know the answer.