Close friendships and romantic relationships have many differences. There are thing people would do with their couple and thing they would only say to a couple.
For example, saying "i love you"or holding hands.
If they give each other nicknames they wont choose cute affectionate things like "honey"
If they compliment each other a lot about their beauty, that is a thing too. Some female friends may compliment each other, but is not a thing that happens too much.
Some friends getting too clingy or possesive about their friend can be mistaken for that if it is a recurring thing during the story.
There are other things, but as a general rule, the key is to understand the esential differences between romantic and nonromantic afection.