Open all of your assemblies and try to move all of your parts to an archive folder. The ones in an assembly won't move.
If you look at the folder in SolidWorks Explorer then you can pick each part one at a time and look at the Where Used info. If you have a lot of parts then this will be a tedious process.
I would do a pack and go of your assembly. To do this open your top level assembly, go File - Pack and Go. you can then select a new folder location for to save your files into. This will save copies of all your files to this new location, you can also include any drawings that you have if you have them. You will now have a copy of your assembly and part files etc in this new location. Close the assembly that you have open. I would then open the assembly in the new location to make sure everything is correct. You can now either compare the files in the two locations in windows explorer and remove any files you don't want from your original folder or delete the original folder and its files and just keep the new folder and files that you created in the pack and go process.
Another way to see what parts are in your assembly is to use the "Assembly Visualization" tool inside of your assembly this will give you a list of the parts and quantities use in the assembly