Or you could try using 2 other cards to draw a horizontal line across the 2 lines for comparison. One card is used to form the right angle and the other is used to slide up to see which line end it reaches first.

Or if you could get a good shadow you may be able to do it with just one other card with the 2 comparison cards aligned and the shadow card tilting down.

Edit: Or just use table corner. 2 cards along edge and line drawing card on other edge.