Depends on the guy, the girl, the situation, the type of eyeing.
As a general rule I don't believe in ogling (only gives her more validation.)
However letting her seeing you noticed her AND immediately following through with casual laid back conversation shows you know what you like and aren't afraid of going after it.
Some like to act shocked, wrap their arms around their chest, put on a jacket, very obviously trying to make you feel bad about noticing that her tits are spilling out of a low cut blouse. Shows you who's a tease or man hating feminist and prevents you from wasting time with her.
As a general rule I don't believe in ogling (only gives her more validation.)
However letting her seeing you noticed her AND immediately following through with casual laid back conversation shows you know what you like and aren't afraid of going after it.
Some like to act shocked, wrap their arms around their chest, put on a jacket, very obviously trying to make you feel bad about noticing that her tits are spilling out of a low cut blouse. Shows you who's a tease or man hating feminist and prevents you from wasting time with her.