Time, mostly.
Though I know that if I feel like it'll make me violently ill, that's generally some serious infatuation, because my body gets all torn up over such inappropriate feelings. Granted, sometimes love makes my insides churn like there's a serpent crushing my kidneys and liver in its coils.
If I feel intense existential dread then it's usually love or a crush.
Whether I find myself building up the other person as more awesome than they are is also a fairly good indication of something fishy going on.
Sometimes you don't know exactly what to make of someone and are doing the dance of just getting to know them from Adam for a bit before one decides.
The only people I know of who advocate becoming friends first are the people who feel that they have to be friends with someone first before they can become attracted to someone else in a meaningful way and people who think that being a "nice guy" can work out for them.
Eh, I'd argue that deliberately conning someone out of money and time because one knows they have a thing for one's self is more intrinsically immoral than just having a thing for a friend who doesn't feel the same way about them.
Besides, once a woman is doing that kind of thing to someone, it's clear that she does not view that person as a friend, or even a person worthy of the most basic levels of respect or fair treatment.