Weight.
How easy it is to cut or scratch it?
Try to stick a fridge magnet on it. If it falls off then your keg is either aluminium or top grade stainless, and does anyone know if they use top grade stainless for kegs? The attraction is weak but they will stick to my Cornies.
Either way, check the legality or may find yourself on the wrong side of Kegwatch, and you wouldn't want that :nono:
