Safer method, jack both rear wheels off the ground, put rear of car on axle stands and release handbrake. With the engine off pick one of your rear wheels, it doesnt matter which one, and rotate it, whilst looking under the car at the wheel on the other side of the car. If the other wheel is turning in the Same direction you have an lsd, but if it is turning in the Opposite direction you have either and open diff or a viscous type diff. And on the subject of wheel spinning and donutting, my old mv6 quite happily diffed around a PRIVATE car park beside antrim lough, with an open diff and who flung dung remoulds, if youve got the power you will lock the diff anyway. Driving on skinheads you have less restistance between the road and the tyre, hence the reason your car would hoop so easily,because it was easier for it to spin the wheels. But at the same time, under hard braking you would have noticed the abs kicking in with less effort too because the wheels would lock easier. I'd weld the diff if it was mine, but at mot time, i find the little monkeys in their blue suits have sense of humour failure when they try to do a brake test, the look of confusion on their wee faces is priceless