I haven't had a male friend in about 20 years. I should temper that by saying I haven't had many friends since then. Before then I'd tend to have a set of 2 or 3 friends for a few years, before wandering off to another 2 or 3.
but one way to tell, I think, would be if you have feelings for them that you wouldn't have for a male friend. I think romantic feelings make themselves known by a certain yearning for the other, a preoccupation with them.
_________________
Of course, it's probably quite a bit more complicated than that.
You know sometimes, between the dames and the horses, I don't even know why I put my hat on.