How to show user that here you have to hover cursor? All we know that if it is something convex it's clickable. How to show hoverable element?
What you are addressing in your question is hover affordance.
Usually, clickable elements have also their hover states, but these point to click affordance rather than hover affordance. In other words: oth design of an element (or: its style) gives a clue thet the element is clickable. One of the popular conventions to show hover affordance is dashed (or sometimes: double) underline of some text.
You can also give user a temporary clue, as described here: http://www.loop11.com/affordances-can-point-your-users-in-the-right-direction/
The other idea, is make element semi-hidden, so that attracts user to point the cursor to that position. Then, s/he is going to suspect that there si something to uncover, which is what you want to achieve. So, a label saying "pull me" (not neccessarily: in words, it may be just proper design) may be a good direction. You may read more about it in this question: How can I make it obvious that a page element is to be hovered over?