The first and most dramatic way to test your web pages is by putting your own browser to work. You should set up your browser to display pages without images. And, you should try viewing your web pages with large fonts and with different color schemes.
While you perform these "tests," remember they are real-life necessities for people who are blind or visually impaired.
To see what a screen reader user would hear, or what a non-graphical browser user would see, you can view pages with images turned off. Use the Web Accessibilty Toolbar : Images > Remove Images. Below is a screenshot of part of the Travelocity.com home page, before and after the Remove Images function has been performed.
With the two screenshots - one unaltered and the other with alt-text replacing the images - you may be able to look back and forth to compare alt-text with the image to check the quality of the text alternative. The Toolbar (of course) offers other better options for doing that. If you use Show Images instead of Remove Images, then the alt-text will be placed next to the image for easy evaluation. Alternatively, you can use List Images which creates an HTML page listing all the images and the code associated with each - there you can see the alt attribute.
Some users view pages with fonts and colors changed, which is not the way the web developer specified them.
Many people with disabilities are not able to use a mouse because they lack hand-eye coordination, because they can't see the mouse pointer or because they lack fine motor control. All pages should be tested with the mouse hidden from the tester's grasp. Use only the keyboard to navigate from link to link and to all form controls.
With Microsoft Internet Explorer or Firefox use the Tab key to move from link to link and around the elements of a form. Shift+Tab moves backwards. With Opera, use the A key to move through links, Q to move backwards. In Opera the Tab key moves between form controls (Shift+Tab backwards). Try using some familiar site in this way (no mouse!). Use the Toolbar (last two buttons) to open the current page in Opera or in Firefox.
The Web Accessibilty Toolbar is really a testing tool itself. You can quickly check alt-text on pages, and if images are missing this required attribute, the Toolbar lets you know with an alert how many images have no alt-text.