The following command will allow you to view debug messages on the ASA for IPsec traffic:
debug crypto ipsec
The debug level would be of your choosing. Higher debug the more information you will see. You can also debug IKAKMP as well.
debug crypto isa
However, if you wish to debug this issue as it happens you would have to wait until it occurs while your debugging on the firewall. I don't think this would be ideal to simply wait until it occurs.
If you want, you could also enable logging to flash on the ASA for the vpn traffic which may provide some information as to why the tunnel went down.
logging buffer-size OPTIONAL
logging class vpn buffered informational
Has this issue recently appeared or has it been ongoing? Have you changed the time until the phase 1 and phase 2 SA's rekey? Do you know what the remote VPN rekey value is set? The IPsec tunnel will agree upon the lowest values for re-negotiation on the security-associations. What is the remote device your ASA is terminating the VPN tunnel to?