Measure slope angle with electronic spirit level!
If you need to check if given surface is straight you can use precise electronic bubble level! It's the most accurate and useful tool on the market! It clearly shows you the angle of the surface on which your phone is. You have 3 types of spirit level to use. One bubble level is horizontal, second vertical. Third is bidirectional.
It's easy to use and very practical tool for checking the slope of every surface. If you don't have physical bubble level - just take your phone in hand and measure the angle of the object.