How do I use scrollTop when trying to figure out sidebar position?

I posted this question yesterday and 开发者_开发问答the answer I go mentioned that I should check if scrollTop is higher than your header height. My question is how do I do this with query. How do I do this when different people have different screen sizes

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$('selector').height () will return the height of the first element to match the selector in pixels. It's all in the manual.

http://api.jquery.com/height/