Well, typically when i catch a girl gazing at me, I do not assume that they are checking me out. A person can be looking at you for several reasons. They could be gazing at you because you look like someone they've met before, they could be looking at something that is behind you, or something or someone close in your vicinity. When I do catch someone, I feel neither flattered or disgusted, because the reason or direction of their gaze is uncertain at that point. Though, it really depends on the situation and the type of gazing behavior that is employed.
In psychology, body language is not black and white. For example, if a person avoids eye contact when asked to tell the truth, it does not mean that they are lying, (although that is popular belief). Looking at a face is associated with high cognitive load. In other words, looking away from a person's face can actually help a person free up more brain cells to say, recall a past event. There is actually no correlation between dishonesty and gaze behavior. Most people believe that they are exceptional at detecting deception behavior, but the truth is most people are horrible.
If a women smiles and looks at you, those aren't necassary signs that they are checking you out. In fact men can often misinterpret these as signals of interest. If I recalled it correctly, women typically smile more often in comparison to men, maybe because this is expected from society (though that may also be a gender stereotype.)