I just did a dyno recently, top of 4th gear so ~130mph. At the initial start the front wheels did nothing for a few seconds (ie, < 5 mph) but at the higher speeds the front wheels never seemed to slow.
On the contrary, when the dyno operator let off, the front and rear wheels seemed to dis-engage on the dyno the same way. I don't know of any way to tell if the fronts had power or not over 100mph, but from 112-130mph if there was no power from the fronts you would think that the front roller would be spinning slower. I suspect the only way to tell would be to read the power levels from each dyno drum, that would tell you what the front drum had going to it. I will be going back to this dyno when I get my FMIC and DP on, I'll talk to the operator about what he may or may not have in his software to show each drum's details.