Astarion: It actually kind of makes sense to me you don't recognize him as a vampire spawn right away even if you had some familiarity with the creatures. The tadpole changes all the rules for Vampirism. Kind of hard to accuse a dude of being a vampire when you first meet him under a very bright sun. He then has no problem crossing water or entering homes uninvited. Even if there are some vampire flags I feel like the oddities would give enough cover to throw suspicion for a little bit.
You are right, since he is able to do all these things everyone knows a vampire is not able to - first and foremost walking around in broad daylight- our characters would not really have a valid reason to be suspicious. He might look like a vampire, but he cannot be one, since he has no problem with the very bright sun.
Taking this into consideration, I think his secret would be a secret for our character, and we wouldn't really need a check. Unless we see that he has no reflection in a mirror
The only thing that bothers me a bit is the visibility of his bite marks. He must have been abducted at night, and in a world where vampires exist, I think he would have tried to hide these scars.
(I have to two chickenpox scars with the appropriate space between them near my collarbone, but in a world where vampires do not exist, I should be safe
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