In order to decide if one is in love, one must have a clear definition of love. Otherwise, one would not know if a given experience classifies as love, hate, passion, and so on. Yet, in trying to establish a clear and possibly universal definition of love, one will inevitably run into problems, for I don't think such a universal defnition exists. What we're left with, then, is something along the lines expressed in Tolstoy's Anna Karenina, where Anna says, "if there are as many minds as there are men, then there are as many types of love as there are hearts." Thus, it is to be noted that in this vein love is a platonic concept. If love is a platonic concept, and there is no universal definition and clear-cut criterion of love to check off, then each will have his or her own definition, conception, and criterion. Still, in establishing a definition, it is important to take into account that love typically refers to a relationship of some kind. It could be a relationship between humans, as in the case where a man loves a women and the women the man, we can say the two are in love. Alternatively, it could characterize a relationship between humans and objects/abstract ideas, as in this man loves chess or theory. Still more, one could love one's self as is the case for a narcissist. So, I think one can have a rough estimation of love as form, but inevitably the 'stuff' of love--that is, the content--will vary considerably for different humans. Another issue, which makes expounding on love a difficult task, is the issue of finding thoughts that express feelings in a meritable way. Indeed, I know many who can never find the proper words to clearly communicate their feelings, and this causes them frustration at times. I myself have no such problem, for I am not capable of experiencing a feeling that cannot otherwise be effectively expressed through thought. Thus, for me the letter is more important than the spirit, and only when the birth of a thought, "I am in love" surfaces, which clearly denotes the experience (love), will I know that I am in love. Thus, from personal experience I think, and by extension what you may feel, is the following. (1) Love_DEF_the spiritualization of passion(s) (2) In terms of ontology, and to answer the main question of this thread, you know you're in love when the thought "I love X" comes into being.
I anticipate criticism. First, that one can think one's in love and not be. Second, that one can be in love without knowing it. In the former case, there is a good chance one does not have a clear definition of love, and thus their personal definition is not descriptive of their actual experience (perhaps it was not really love they experienced) but all this means is that they lacked a clear definition of love to begin with. In the latter case, which supposes it's possible to be in love and not know it--well I don't know because all I know is thought. So perhaps an NF (or someone else) can explore this as a possibility. I suppose also that this is really at the crux of this thread, as the question you raise is one asking for evidence--signs if you will--that indicate that one is in love. This opens up a range of questions involving definitions, concepts, reasoning, personality, and so forth. But I stick to my fundamental point, that I can only be sure that I am in love when my love has become a conscious thought. Thus, for me knowing I'm in love is a question of ontology. Anything besides that is a muddle, where lines between love, lust, passion, happiness and so on are thin. And I know one thing for sure: that I don't like playing chess on a board where there is not a sharp contrast between the different colored squares. Thus, I have answered the question as straightforward as I'm able, and I'll let the feelers (or whoever) delve into the wishywashy stuff, often a question of conscience rather than logic, which I dare not get into.