Use Vischeck to test your own templates. To do so, you need to "save as" a PowerPoint page as "PNG".
A side-benefit of this test is that you get sense of what happens if someone prints your presentation on a black & white printer. (But hey, the B&W white test is the easiest of all: print preview)
Somewhat related: an earlier post about designing presentations with people suffering from dyslexia in mind.
Via Richard Garber. A more elaborate post on Vischeck and PowerPoint in this post on the Indezine blog.