Because sometimes, you just have to read the signs.
Reason
If a girl you only knew in passing contacts you out of the blue for a specific and legitimate reason (as opposed to no reason or some flimsy excuse), there is no need to think too much into it. In all likelihood, the girl is only looking to you for that specific reason and nothing more. And because she has no underlying intentions or feelings towards you, she can be friendly and open towards you while maintaining the status quo. However, if the whole thing was a ploy to get close to you, the girl would soon start nudging her way more into your personal space by asking you if you wanted to go for a meal, or other innocuous "date" activities, together.
Reaction
If you like a girl and start testing the waters by throwing her cheesy lines or even obviously stating that you have been observing her, then it makes sense to check her response. Was it positive? Was it negative? Did she rebuff you, ignore what you said or did she accept your offer/compliments? Girls are not idiots and they are not oblivious. Acting oblivious can also be a form of rejection. Anything short of a positive response and she prob'ly isn't interested.
Interaction
A.
If you disparage something a girl does, which you haven't tried out yourself, chances she'll like you diminish exponentially each time the topic comes up and you can't stop yourself from making the disparaging comments. It doesn't mean she likes you if she tolerates this - it just means you're just a friend and it's not worth getting into an argument over.
B.
If you whistle, making sucking or kissing sounds a la an Ah Beng to attract a girl's attention, the Like-O-Meter will dip, dip and dip. Well, unless she is fully aware that you are a bona fide Ah Beng. But she might ignore this if she sees you as just a friend - and she's not your mother.
C.
If you talk down to a girl even if you don't know her age, it pretty much shows your need to feel superior and/or you don't consider her your peer. Independent and strong-willed girls are unlikely to go for this, particularly if they are in fact older than you. But she might indulge your overbearing complex since you're just a young friend.
D.
If you comment on a girl's food intake even though you have in fact never eaten a meal together, it may or may not be another nail in the coffin. Some girls don't give a shit, while others are très sensitive about their weight and calories. But if it's just a clueless friend, she can take it as nothing more than a thoughtless comment.
How are the signs looking?
