Depeneds really, theres no real rule.
Some girls will never tell you, and your chance will just pass on by.
Some girls will pay you way more attention, and if you look carefully, youl see that its more attention then theyre paying anyone else.
Some girls just go ahead and tell you, these are my favorite, but then im a simple guy, I hate having to chase, it bores me and im likely to wonder off.
Just go with it man, theres no harm in telling her that you like her or inviting her out for a drink, a girl who doesnt like you like that will simply turn you down, but one that DOES like you will be all for it and probably happy that she didnt have to ask herself.