Question

How do I find true 90 degrees

Answer 1

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.