How can you know if someone is transgender?
There are some ways in which it is possible to make a guess if someone is trans or not. A broader jaw, more muscles, their height, broader shoulders.
Possibly poorly-applied makeup, a slightly off-putting dress sense, or possibly the condition of their hair. The way they walk in heels, the tone of their voice, or the condition of their skin.
The curve of their hips, or the size of their breasts, or if they have any facial hair. If they have a penis or a vagina. These are all possible clues that someone might be trans.
But there is no definite answer I can give you beyond this because both cis and trans women can and do have all of these features, or none of them. The presence of these factors, or lack of them, is a pointer, but not a guarantee. The best way to know is to ask someone.
Of course, your question also has another problem that I’d go amiss to address. You ask, “is there a way to tell if someone is trans,” and then say, “it can be hard to tell if someone appearing feminine, is, in fact, female,” and this heavily implies that trans women aren’t actually female.
I suggest you think long and hard about your views on transwomen, transmen, and the LGBT community in general, because trans women are just as many women as cis women, and thinking like yours is dangerous and regressive. We are real, and we are women.