Somewhere in the possible near/far future when there might not be any major fiat anywhere. What will Bitcoin's purchasing power be measured in?
If in this hypothetical scenario there weren't any other unit of account available, it would be necessary to use some kind of consumer price index.
Fortunately, besides fiat currencies, there is also gold as unit of account. So, in case all fiat currencies are gone, you could use gold to measure the purchasing power of bitcoin.
It's typical to measure the purchasing power of currencies using something like the Consumer Price Index, so this is probably would be used if Bitcoin were to become a dominant currency.
Basically, you would develop a "market basket" of goods and services that people commonly buy, and perform some sort of survey to determine what these goods and services currently cost (in Bitcoins). Then take some weighted average to come up with a single number that, roughly speaking, measures the overall Bitcoin price of goods, or equivalently, the purchasing power of a bitcoin.
There is now a BitcoinPPI (Bitcoin purchasing power index) available:
It uses the BigMac index published by The Economist and calculates a global weighted index and a local index for 41 countries.
Disclaimer: I have no opinion on the taste of BigMacs ;)