tirpitz's way is definitely the best and most accurate way to determine if you have posi, however, the other ways are look for the G80 which means you have posi, or you can block both front wheels, put the car in neutral with the engine off, don't set the parking brake, jack up the rear of the car by placing a sturdy strong jack under the differnetial (center it on there well). now go to either rear wheel, and turn it, the other rear wheel shoud turn in the same direction, if it does, you have posi, if it turns in the opposite direction or doesn't turn at all you dont have posi.
what posi does is sorat like the all wheel drive commercials you see, it transfers power form the wheel that slips to the wheel that grips, and if neither wheel is gripping then they both wind up getting equal power. It does have some margin of slippage included though (limited slip not no slip) becasue when you turn your car while its moving, the wheel on the inside of the turn moves slower and slower than the wheel on the outside, the degree of difference in rotational speeds is directly proportional to the tightness of the turn. People who dont do burnouts or drag race will never really need posi unless you live in an area with snow or rain and your tires wind up slipping. Drag racers like posi because they can essentially get twice the power to the rear axle when launching their car before losing traction. in the snow, you always see car stuck with one wheel spinning and the other wheel just kind of sitting there, and usually its the one on pavement, the spinning one is in the snow, posi will take the power away from the spinning wheel and give it to the other one. its the same in the rain. for those who do not drag race or to burnouts, posi makes it just a little harder to get the car stuck in bad weather, and for those who drag race, they can launch harder, and burnouts look cooler when both wheels are smoking and 2 patches of rubber are being left behind on the ground.