The discrete (separate and distinct) nature of these feelings should distinguish itself slowly, over time, as you truly come to know the significance of another person, I.e. who is this person, inside? The initial feelings of attraction that we as humans feel, when encountering another, do come in different forms; but they can become a bit jumbled up in the beginning stages of ‘getting to know someone’. Again, when truly making an effort to foster a relationship with another person, the nature of that relationship should reveal itself naturally to you with time – as a casual acquaintance, a close friend (strictly platonic), or as a romantic partner.
Also, Misha: You raise an excellent point when you say that you have to ‘think about it’. That is a key component, as it’s easy to get swept up in an initial whirlwind of emotions when encountering another. Far too often, folks are led, blindly, by these initial feelings of attraction. Only later does the mind (source of our thoughts) catch up, at which point many folks ask themselves, “What was I thinking?!” The answer to that would be, “That’s just the problem; you weren’t thinking.” 😬