Hi,
If you have logs going to a Syslog server you could go through the logs from a longer perioid of time. This might require certain of logging to see what connection has been formed through the firewall.
You could also configure packet capture on the firewall itself to capture the traffic between those networks (would have to do this on the firewall LAN interface) and then see what type of connections are being initiated and from which end of the L2L VPN.
Packet capture with a separate computer in between would naturally provide chance to save more capture data. ASA has limitations to buffer size but is pretty easy and fast to configure.
- Jouni
