Hey Blue - there are a number of identifying features between the two 'front-loader' 8 and 9-inch axles, but the simplest is the "bottom bolts" trick. See KevinStang's page
for some pics and more info.
As far as which is better, the 8" in lighter and will hold 400 hp on street tires reliably, and less on slicks and harsh treatment. The 9" is heavier but can hold more power stock, as well as be built-up to take tremendous power, and parts are everywhere. While there are other rears that can be built to take similar or even more power, they are considerably heavier for the power they can hold than the legendary 9". HTH