the steering wheel is spinning on my bmw x5 v8 2001, but the tires aren't moving , happened whilst parking.. the shaft inside the vehicle is turning with the steering wheel but nothing happens, could it be the joint on the other side of the firewall or in the steering box. And how do I check it?

I think that's what happened to my van and it was the carter pin at the bottom of the steering column.

Did it happen whilst parking or while underway

Just starting, went down the small hill from my driveway and had to roll into the Park's parking lot.

Check the lower universal joint on the steering column. You will need to remove the boot for access.