When the attraction has turned completely sexual in nature (I assume un-married). I.e. the other becomes an object. I think any time bodily pleasure becomes the centered thought you are pushing into sin territory. Another thing to think about is if you place more emphasis on what you are looking at as opposed to who. If your mind begins looking at your significant other’s outsides in an overriding manor to his or her insides
Rule of thumb: if you can think of God while being in a state of attraction, you are in the Good, but whenever the attraction takes on an over riding state to thinking about God i.e. you would be uncomfortable sharing your current feeling/thought with God in a “casual conversation”, then you may be pushing into bad territory. For example: I can think God and she has nice hair together at the same time without a problem…but replace that with God and engaging in pre-marital sex, then the thought of God suddenly removes itself from my mind and I see my conscience as choosing my desire over that of God’s i.e. they are not in line together. That may not be practical, but you left the question very open.
In terms of bodily response:
Attraction - happy, wholesome, real – I think about the encounter in a positive light
Lust- Dirty, fake, foolish – feeling like the thought I had was without purpose and should have been avoided