It really depends on search volume vs SERP in combination with cost per click If you have a keyword that gets 10 searches per day, and you rank number 1 with an ad that will pay say...10 cents per click, Not gonna make so much money. The more money you make really is dependant on the size of the niche, and the amount of money advertisers feel they feel they can pump in to get a decent ROI.
create channels for your adsense codes for each website that way you will be able to tell which website is bring more.
Ahh...nevermind I interpreted you wanted to be able to gauge how much potential each site had...err... Use channels then for the real answer . Ignore the above post.
When you creating the Google Adsense ads, in 2nd step, it says All channels. You can create channels there and assign to different ads block. After that, your income will be shown according to your channels.