Place the spring on a flat surface next to a square. Measure the height of the spring, and rotate it against the edge of the square to measure distortion. If spring height varies (by comparison) by more than 1.5mm (0.059 in.) or if distortion exceeds 1.5mm (0.059 in.), replace the spring.
In addition to evaluating the spring as above, test the spring pressure at the installed and compressed (installed height minus valve lift) height using a valve spring tester. Spring pressure should be within 1 lb. of all other springs in either position.