To the best of my knowledge all current Canon "L" lenses have ring type USM focus motors, other than the TS-E lenses that are designated "L". They have no focus motor of any kind as they are manual focus only lenses.
If the lens has ring USM then you can move the focus ring at any time without fear of damage. If the lens has Micro-USM and the focus ring does not move during autofocus it is also safe. Lenses with full time manual focus will also allow you to continue to turn the ring even when the focus is all the way to Minimum Focus Distance (MFD) or infinity.
If the focus ring moves during autofocus or if the ring hits a hard stop at MFD and infinity, then you should only move the focus ring when the lens is set to Manual Focus.