A method of evaluating edge detector output is proposed, based on the local good form of the detected edges. It combines two desirable qualities of well-formed edges -- good continuation and thinness. The measure has the expected behavior for known input edges as a function of their blur and noise. It yields results generally similar to those obtained with measures based on discrepancy of the detected edges from their known ideal positions, but it has the advantage of not requiring ideal positions to be known. It can be used as an aid to threshold selection in edge detection (pick the threshold that maximizes the measure), as a basis for comparing the performances of different detectors, and as a measure of the effectiveness of various types of preprocessing operations facilitating edge detection.