I really have no idea. In my current relationship it really just crept up on me (after having a couple of really "passionate" relationships). Probably when I told DP that I was taking a job in another city and instead of just saying "see-ya" he offered to drive down with me and leave me his car (he had a company car). We maintained a long distance relationship for 14months before moving in together.
You WILL Know, believe me.
You will think about that person all the time.
You will be happy like you never have.
You will know, from the signs you get and esp feel.
With my husband and I it was obviously lust initially (well we were 16 and 17). It grew stronger over the years to a much deeper love which I still feel although my husband died two years ago.