A concave lens is thinner at the center that at the edges. So the light beams passing through the lens are spread out or diverged. Therefore, the concave lens is called a diverging lens. The image formed by a concave lens is virtual and diminished. Since a concave lens will not produce a real image, a convex lens is used to measure its focal length. There are two methods to find the focal length of a concave lens: (a) Lens in Contact Method and (b) Lens out of Contact Method.
This video explains how find the focal length of a concave lens using a convex lens by: (a) Lens in contact method and (b) Lens out of contact method.