Actually, I found an easier way to do it this afternoon. If you're reading a work, click on the word you're interested in.
Let's take the beginning of Cicero's Pro Roscio Amerino for example. It starts off: credo ego vos, iudices, mirari quid+ sit quod+, cum tot summi oratores hominesque nobilissimi sedeant, ego potissimum surrexerim, is qui neque aetate neque ingenio neque auctoritate sim cum his qui sedeant comparandus.
Click on, for example, credo, and the parser's little window pops up. It has a frequency table, and if you click where it says Cicero and Max Inst. it lists all the examples of credo and other forms of the words in Cicero's works on Perseus.
I take it all back. Perseus still rules. Tho' it doesn't have Cornelius Nepos yet. [hint hint]