How can I determine if a div is scrolled to the bottom?

You're pretty close using scrollTop == scrollHeight.

scrollTop refers to the top of the scroll position, which will be scrollHeight - offsetHeight

Your if statement should look like so (don't forget to use triple equals):

if( obj.scrollTop === (obj.scrollHeight - obj.offsetHeight))
{
}

Edit: Corrected my answer, was completely wrong

In order to get the correct results when taking into account things such as the possibility of a border, horizontal scrollbar, and/or floating pixel count, you should use...

el.scrollHeight - el.scrollTop - el.clientHeight < 1

NOTE: You MUST use clientHeight instead of offsetHeight if you want to get the correct results. offsetHeight will give you correct results only when el doesn't have a border or horizontal scrollbar