If you're on Adsense only (as opposed to Hubpages ad program) then you can find this out using Google Analytics. Mine showed me that my most popular hub earns almost nothing! My earnings have actually come from my less popular hubs! However, the Google Analytics only tells you about the money each of your hubs are making FROM ADSENSE. It won't tell you about how much is made off the Hubpages Ad program.
To look at your Analytics data, you need to make sure you have linked Google Analytics to your Hubpages account - this is something you can do from within your Hubpages account. It's in the same part of your Hubpages account as signing up for Amazon, etc etc.