How to record and judge the sign of tilt, that is the sign of configuration error, in one interferogram where the fringes are frozen is presented. If a flat surface is tested by a Twyman-Green interferometer without an imaging lens, not only usual straight and equally spaced fringes but also the extra curved fringes are caused. We can judge the sign of tilt by knowing which direction the extra fringes are curved. The application of this technique to a lateral shearing interferometer is also shown.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.