Think of the hamstrings as the puppet strings for the feet. When the lateral one pulls harder the foot turns outward. If the medial ones are stronger the foot turns inward. When your client is face down, bend the knee at 90 degrees and level the foot. Whichever way the foot is pointing that is the tight hamstring. Slight toeing out is normal but more than that causes stress on the knee due to the rotation of the tibia. So bottom line, lengthen only the tighter hamstring.