Run tcpdump filtering for the IP address of the VPN peer. (assuming 19.168.1.1 you attempted filtering for is an internal host).
On the outside interface if the firewall you should see ESP packets to/from the IP addresses of the two VPN gateways, these are the encrypted and encapsulated packets. Possibly a few packets on UDP/500 for periodic key exchanges / updates, and a few when first establishing the tunnel.
But if you see 4-5 packets on UDP/500 every so often (maybe 30 seconds or so), and no ESP packets, it's usually down, there's a problem. (and if you don't see anything, most likely you have a mistake in your tcpdump command).