Runflats have "Runflat" written on the sidewall
....absolute bugger to get them on / off a rim due to the thickness of the sidewall.
If a tyre does go down, it will look a little low. The error on the iDrive comes from the ABS sensors. As the tyre loses pressure, the ECU picks up the size difference between the good tyre and low tyre (low tyre travles more revolutions) and flags up the error
...of course the other way of checking is to check the pressure.