This question demonstrates why I believe there should be a hard division between QA and Test. QA is about standards and process, testing is about validation and verification.
As part of the QA process, you look at the documents in terms of style, format, conformance (BS5750, ISO9001, House rules etc),etc etc.
As part of test, you check that the information is accurate and complete (as already mentioned, carry out the activities described in the document).
In order to test the documentation- may be Installation or user manuals or Release Notes, I guess you first of all need to formalize the process for it.
By Process i mean, answering the following questions-
1. How are you going to categorize the documentation defects ?
2. What would be the defect tracking mechanism ?
3. Who will be responsible for the defects in documentation ?
After these questions are answered- you should create a checklist for what to look for in the documents. Checklist are a very important tool that will help you streamline the document review process.
Like for Installation document the checklist may contain-
1. As vinod said, testing the installation as its mentioned in the doc with the actual software.
2. Checkout that information is consistently mentioned across the various sections of the document.
3. There is no repitition of information in the document.
4. The information present in the document is relevant i.e. at no point is it discussing any topic that is of no use.
5. All the mandatory sections pertaining to Installation i.e. Installation steps, Uninstallation steps, platform supported etc. are there.
The benefit of following a checklist is that you ill have your test approach documented and you can present it to anyone when asked for it. In absence of a checklist you will always be giving vague answers in case someone asks you how you test the documents.
Also, the same document can be used in other projects in your organization, thus forming a standard.