How do I find true 90 degrees
If you are talking about geometric construction or laying out points for building a house, you can take advantage of Pythagoras theorem for a triangle with sides 3 and 4. After the hypotenuse has been adjusted to 5, the right angle will be a true 90° angle.
Conversely, for a given angle, measure the adjacent sides and the hypotenuse and check by Pythagoras theorem.
If this is not what you are asking for, we'll need a more detailed description of your problem.