Depending upon how you will be using a wiki for your project, you may want to evaluate the wiki or you may be assessing student learning and quality of collaboration through using the wiki or both.
For instance, if you are using a wiki to more easily enable small group collaboration on a writing project, you will probably create a grading rubric of the writing project and also a rubric of group collaboration. (A rubric on product and another on process.) You may want to create three rubrics for the collaboration element (self-, peer-, and teacher-assessment) and also conduct several formative assessments of the collaboration, to see if it improved, stayed the same, or became worse.
(If you are using a wiki for your entire course, then list some of your learning objectives and how you will be using the wiki to realize these objectives.)
If students will be creating a wiki as a website to showcase and publish a final project, then you will most likely be evaluating students on the project itself (again through a rubric) and also on how they used the wiki to present their project (a rubric detailing elements of the wiki that are important, such as font color, layout, navigation.) Is this starting to make sense?
Here's a page on pbwiki that isn't perfect, but provides some additional insights on assessing a student's wiki: http://educators.pbwiki.com/StudentWikiAssessment
So, my advice is to create your wiki assessment just like you would create any assessment . . . based upon the learning objectives, what you want your students to be able to do after they have completed the instruction. However, since wikis also invite and almost demand collaboration, you will most likely want to include a collaborative rubric, allowing them to self-assess, as well as assess the others in their group. They should do this several times throughout the collaborative process. And this assessment can be done electronically, collecting the information and easily averaging the scores for you through a spreadsheet, for instance.