You'd probably have to use the onmouseup event (I presume the user has to click and drag whilst holding the button down?)

When the user has released the mouse button, get the position of the element using offsetHeight and offsetWidth.

Then you can compare these results to those of the correct answer position. If they match within X pixels (so the user doesn't have to drop it in the *absolute* correct position), then they've got the answer right!