Hi,
Not sure there's a nice way to look at it.
A VPN tunnel is in use if there's interesting traffic flowing through the tunnel.
You can check that there's traffic flowing with the command ''sh cry ips sa'' and check the encrypt/decrypt statistics incrementing everytime.
The IPsec SAs have a lifetime (28000 seconds) by default normally.
If there's no traffic flowing in that time... the SAs are torn down (tunnel is no longer up).
I imagine that you might log VPN traffic using either syslogs or NetFlow, but not sure if that's what you want to see either...
Federico.