Jack the car up with the Trans in Park for an auto or in gear for a manual and the parking brake is not applied, try rotating a rear wheel. If the wheel turns freely and the opposite wheel turns in the reverse direction it's an open differential, If you can not rotate the wheel at all, or can with a good bit of muscle (The other wheel will still turn in the reverse direction), it's a sure grip.
Of course the burnout test is still the fun way to check:![]()