28 Spline or 31 Spline Axles???

We all know that the 31 spline axles are stronger than the 28 spline axles, but how do you tell the difference without a lot of work? For 1969 and 1970 axles, it is as simple as removing one of the rear wheels. If the end of the axle has a rectangular shape, it is a 28 spline. If it has three dimples (like partially drilled 3/8 inch holes) in a row, it is a 31 spline axle.