A: No. If you’re confident that you can always tell… pause for a moment to ask yourself how you could possibly know that. Suppose you were wrong, and you didn’t notice that somebody around you was trans. How would you notice yourself not noticing?
People are known to make mistakes in both directions. Nickelodeon fans, who watched Michael D. Cohen’s acting for years, did not know that he was trans until he told them. Cis women have been harassed in California, Connecticut, Michigan, and Texas because someone mistakenly thought they were trans.
Cis people have no independent way of judging whether they can tell that someone is trans. Trans people have more relevant knowledge to draw on: we notice how cis people treat us, and how their behavior changes based on their perceptions of our gender. Trust us: you can’t always tell.
A Trans 101 Blog 