Anyway, can a doctor tell someone receives anal penetration by looking at it? Or even feeling it?
I very much doubt a visual inspection will reveal anything.
However, in older books - talking about the 1960s and prior, even pre WWI - I do remember reading that doctors inspecting men, often made a point of noting the looseness of the anal sphincter. Looseness being an indicator - but not definite proof - of anal activity, and moreover, those times being as unenlightened as they were, an indicator of homosexual anal activity (this from before the term gay was used). I believe that in military induction exams back in those times, it was thought to be an indication of possible homosexual inclinations and experience, something to be on the lookout for in recruits.
I also remember reading about this in comparative ethnic anthropology books from before the war. Sample individuals from tribes and ethnic groups, besides being photographed nude, were also checked for their possible homosexual practices by testing the elasticity of the anal sphincter. It used to be a job with perks if you were an anthropologist back in colonial times.
So anal looseness, being easily penetrated, used to be something that sent a signal, something to be on the lookout for, maybe even suspicious of, all depending on a doctor's view on sexual morality.
Nowadays I suppose it shouldn't make much of a difference one way or another. But then again, depending on the doctor's own ideas on morality, it may give them suspicions. Don't forget that a large number of doctors, nurses and medical personnel still believe that homosexuality is a sort of disease that can be cured. There was a post on that subject on Zity recently and I personally found the percentages of docs/nurses in the EU who believed this to be horridly high.
So, while a doc probably can't deduce anyone's sexual proclivities from such an exam, if it were me, I'd wonder what your doc was trying to communicate by such a remark. From what you've described, to me, it sounds like a sort of approval in some way, maybe sexual, maybe just health-wise, maybe because it went so easily for her compared to other patients she's seen.