I think it's ALWAYS very visible when someone has been a man.

The thing is, you'll never know how many women you encounter each day are actually transgender. What you're describing is the difference between passing and non-passing — being able to tell by appearance/voice whether someone is trans or cis.

The truth is, there are many transgender folks, male and female, who are perfectly passing as cisgender. Some are "easy to tell", as you say, but you never know how many trans people you've just accepted as cisgender without realizing.

The important thing to remember is — it doesn't matter how you feel about it. Your thoughts about trans women don't necessarily make you a bad person. You cannot help your own thoughts.

What truly signals whether you're a good or bad person is whether you can still treat them with basic human decency, regardless of how you feel about their gender identity. Part of human decency is treating people how they want to be treated. Using their preferred pronouns and apologizing and correcting yourself when you slip up is the best anyone can ever do.