Matthew:
thanks for the quick response.
I wish I had a ready answer to the question!
I think it's one that has been ignored as being too difficult for quite a while. I can see why they tried to correct that when turbine engines came out, but given that we've had engines lose compression since the dawn of the piston engine, why no-one has come up with an easy way to tell, on-board, if this is happening is an interesting question!
Your point about measuring power required to hover is a good one, but does require some discipline and knowledge of conditions.
Given that I just heard of an instructor who didn't even know where to look for the empty weight of his helicopter, I'm not hopeful we could ever get accurate weights as a starting point!
Still looking - I'm sure someone will come up with some good information.