The lack of a tail hides his Malk heritage whereas his large size and slow aging (Mister is 18) do not.
Being 18 doesn't prove anything, the last few cats who have consented to share my space have lived to eighteen and beyond fairly hail and hearty, mice, rats, and birds, beware.. I have friends with 20 year old cats, Harry does mention that Mister is beginning to show his age..
Harry did not ask BOB (or Bonnie) about what being an starborn meant. They are literally a source of knowledge, and he did not ask. He did not ask Eb either. And yes, he asked LTW and Bigfoot but that was only in PT/BG, when he was aware of him being a starborn for many years before. He could ask Mac too.
Harry has only known about it the last few years, and he has been mostly dead or very busy all of that time. Bob has been with Butters the last few years and Bonnie thinks milk is alive and cannot get her pancake recipes organized yet. Eb so much as told him in Peace Talks that he wasn't going to tell him, he wasn't allowed to tell him.
He did not ask Gray more about Margaret either. In fact, I do not think he even asked Thomas how Margaret was before she left Raith.
He might be afraid of what he finds out. He has a picture of her that he is holding close..
I think now that he realizes being a star born is rather important, and Eb did tell him some things, I think he will find out more as the story progresses.
What I mean about Harry being a detective is not that he had the ways to find answers but that he should be naturally curious, as a professional trait.
But it is one thing to be curious, which Harry is, and obsessed, which he really doesn't have time for.