We orient the concave mirror, at a very distant source of light (object). Then we use a small screen to catch the reflected rays. Adjust the screen so that the image is well formed inverted. the screen is adjusted to be around the principal axis of the mirror.
1/f = 1/v + 1/u
Focal length f of the mirror is -ve, as it is measured against the direction of light rays..so image distance v and object distance u from the pole P of the mirror are also negative.. hence the equation can be used to find the focal length of the mirror.
If we can measure the radius of the sphere of which the mirror is part of, then
f = R/2.
If we can send light rays parallel to the principal axis of the mirror, then they will meet the principal axis at the focal point. This is another way to find f.