It's very difficult. It's not much more than a very rough estimation of 'do you think much slipping has taken place, and vaguely estimate how much?'. The answer can only be Good/Medium/Poor. Most estimations seem to be rubbish, and always exaggerated. It is an estimation of runway slipperiness, not taxiway.
We were in a Classic 747 following a domestic DC9 into JFK when ATC asked the DC9 that question- plenty of snow and ice about. After landing, he said 'Poor!'. We looked at each other in horror, with 30 seconds to go, and selected Full Autobrake. As soon as we touched down, braking was so effective our arms were thrown forward, galleys were crashing and banging, and our noses were almost pressed into the windscreen! I realised then not to listen to such daft reports again- a pilot report is a waste of time!