Spanish, with expressions like “ellos se miran” can be a little confusing, because, without context, that sentence could mean that a) there are two people, and each one is looking at himself (reflexive), or that b) each person is looking at the other (reciprocal).

English makes this easy:

To say that Alice is looking at Alice and Bill is looking at Bill, we say:

They are looking at themselves.

To say that Alice is looking at Bill and Bill is looking at Alice, we say:

They are looking at each other.
They are looking at one another.

You see how simple?  The reflexive pronoun guarantees we know that the action is reflected back on the subject, whereas the use of “each other” or “one another” guarantees we know that the action is going back and forth.

Could Bill and Alice be falling in love with each other?