There are many ways to evaluate student's performance on a webquest.  Most produce some kind of project or written work which can be graded.  Many webquests provide time for discussions, like this one does, PowerPoint presentations, or other times when students can be evaluated on their participation and performance.  Rubrics are often used to evaluate students' performance.

Take a look at this page for some ideas on rubrics.

http://projects.edtech.sandi.net/staffdev/tpss99/rubrics/rubrics.html

How will you evaluate your students' performance on the webquest you have selected?