I don't know about the magnetic bit, but you are correct in that not all stainless is magnetic.
For the weight differences you are talking about you don't necessarily need scales, you could knock up a balance. I must have done this a bazillion times as a kid. Knitting needle, chopstick, heavy duty drinking straw [a rod], and whatever pair of things you can improvise two light cups from [couple of empty yoghurt pots, tin foil moulded shaped into cups, whatever.
1] three bits of cotton thread poked through equidistant holes about the circumference of the cup and united at the other end.
2] Tie the united ends to of the strings one to each end of the rod.
3] Tie a third string to roughly the middle of the rod to dangle it by.
4] Its rough and ready but you can fiddle about with the dangling string till you find the balance point.
It was surprisingly accurate enough to set a lump off against known weight. A half pence coin weighed 1/16 oz, a 1p=1/8oz, and a 2p = 1/4oz. Even with a crude device like this it would be very apparent if someone had taken a liberty and smoked more off it than they should. With your 3oz vs 6oz type difference you could bet a finger on your findings with confidence. Now. What do US coins weigh.
