The outputs of CD players are RCA cables at line levels, so you can hook it into any kind of amplifier or preamp or reciever etc that has line level rca inputs. Just don't put it into a phono input, that might distort the sound and damage some equipment.
As far as testing a mixer without CD players, you can hook up any rca output into it to test it. If you have a headphone jack to rca converter(not hard to find if you don't have one), you can hook up your laptop or mp3 player into one of the inputs and try it out. You can also use the output of something that already has rca outputs, without any adapter, such as a dvd player or video game system.