I'm not sure about checking for bonjour errors on a Mac, but on Windows they are all in the event viewer. Every time I restart the bonjour service or itunes, I get this:
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 24
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 23
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 22
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 21
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 20
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 19
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 18
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 17
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 16
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 15
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 14
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 13
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 12
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 11
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 10
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 9
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 8
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 7
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 6
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 5
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 4
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 3
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 2
Error: Bonjour Service
ERROR: handle_resolve_request bad interfaceIndex 1
Googling this error will lead you to issues with the way it's trying (or NOT trying) to use iPv6. I've completely wiped v6 from my machine and that did nothing. I give up. I will either remote in and restart my machine to get it to sync (works sometimes) or directly connect with a cable to sync. either way it doesn't work as advertised anymore and it's a huge disappointment.