MateXpert is a tool that allows you to identify mating problems in an assembly. You can examine the details of mates that are not satisfied, and identify groups of mates which over define the assembly.
You can also use mate callouts and View Mate Errors to help identify and resolve mating problems.
Versions of the software after SOLIDWORKS 2001 may find a mating error that went undetected in earlier versions of SOLIDWORKS. These are primarily conflicts between mates and in-context features. Remember that when this error occurs, it is important to understand that the mate problem is not a new one. It existed in earlier versions of SOLIDWORKS but was not detected by the software. MateXpert can help you identify these errors.
See What's Wrong for more information on identifying errors and warnings in parts and assemblies.