Typically you see sync failure when you start to get user complaint that a co-worker saved a file but they can't see the file themselves.
1. First, always login to the web portal since the web portal has the file and folder view that is closest to the files in the cloud. (As compared to Windows client, windows client has caching and various WAN optimization so the view maybe a combined local/cloud view)
2. With the cloud view now clear, you can decide whether it is because a co-worker didn't upload the file successfully , or the file is in the cloud, but you can't see the file successfully and zoom in on each case.