I find the best way to do it is let the professional photographer take the photos first then get the cast member to take whatever photos you want of family or just children or just adults etc then you can compare yours and the professional ones. I always keep mine but if there is a particularly nice professional one then you can get this too.
If you don't want to pay the price for lots of photos taken professionally then collect all the photos on the photo pass card and take them to Town Square Photography in Disneyland Park or Studio Store in the Studios park and they will compare all the photos taken on that day so you can make your choice but remember the more you buy the cheaper it becomes!