Your costliest AdWords keywords can give you some light. Otherwise, you can go for health related, home business, loan, debt consolidation, credit, web hosting related words.
Do a Google on 'high paying AdSense keyords' and you'll find loads of sites giving their 'expert' opinion. What they're really doing is loading their own pages with these words to try and boost their own earnings - sneaky, huh?
Well an Adwords account is the best way. its pretty easy to find high paying keywords but beating to competition and getting traffic is the hard part.