The back of the CD box will give some clues, but you can't beat an in-store test drive. Enter a few topics and key words, and check which program has the most concise information. Award extra marks for good photographic content and on-screen layout. Almost as important as content is simplicity of operation. If the kids find the encyclopedia tough to use, they'll be getting you to do all their work.
Most programs are straightforward, but some are easier for younger children to use than others.
Some programs cater for users with high school-level literacy, while encyclopedias are frequently bought by parents of primary school children. Take a look at the words in screen shots on the box or, better still, during the test drive ask the kids to read the words and sentences. Some encyclopedias excel at explaining the basics, while others assume you already know what the topic is about move rapidly on from there. If the kids love to explore, they'll appreciate a program with all the frills. If they want the answers without the fuss, go for one that's hard on facts, even if it has fewer trappings.