How would I check what clients need to download when they join my server?
Well, a simple way which I do every now and then to check and make sure our FastDL is working correctly. Re-name or move your current addons, models, materials, and sound folder, then restart GMod and join your server, it will re download all the files, If you have FastDL rejoin once more to see if it forces you to re-download any files, if so upload those… even if you are not trying to test out FastDL this method works decently for seeing which files need to be downloaded.
You could also check to see which addons have their own resource lua script that will send files to clients (resource.addfile), many things like wiremod, or gun packs and such will have them.
Let a friend of yours join with a fresh install of gmod. Done it a couple of times. Just to get it 100% working.