When someone says what gender they are, I roll with it. Any trait that doesn't apply to societal gender norms doesn't make anybody less of a man or woman. I've seen buff as **** men paint their fingernails and I know women who are plenty capable of kicking my *** in real life just on strength alone. Are they trans? I don't know, but I don't think that's important. It shouldn't be important, right? It's they're gender that decides what pronouns I use, anyways, and I know it's a sensitive subject. On the other end of the spectrum, a lot of cis women I've met are very insecure about looking like a guy. If I went about and assumed as such in anyway, it would devastate them. I also know some men who don't like "looking girly" or something like that. Yes, they're insecure, but that doesn't change the fact it's rude to assume and you have no real basis to prove it as fact.
As a result, I think it would be rude to assume if someone is trans, similar to how it would be rude of me to assume you are a guy given your physical traits.