I don't think I've ever been asked if someone else was trans, but I have been asked if a trans friend had "the surgery" yet. My response was "I don't know, I don't ask her those kinds of questions."
So maybe if someone asks "is that person trans?" you could say "I don't know." Lol. It's not true, yeah, you know he is trans...but eh. I think saying something along the lines of "it's not my place to say" pretty much confirms their "suspicions." Honestly, if they asked him and he said yeah, and they somehow found out you lied about not knowing - then say, "well, it's none of your business to ask me that in the first place, and I'm not going to out my friend without his permission, so yeah, I lied."
I've encountered people like that though and it reminds me why I've come to hate a lot of people - those who are educated enough to recognize trans symbols and whatnot, but apparently lack the education to know that it's not their business and not appropriate to assume someone is trans. Do they look at all the men they see on the street and think "hey, I wonder if that guy is trans, I totally need to know so I can be a good ally to him."