Besides what jjohnston suggested, you could always remark out their cryptomaps and see who complains. :)
It might be good to take his suggestion for a week or two and the communicate to your users that maintenance may affect their VPN connections and be ready to restore them in a moment if need be.
Going forward, it might be useful to give their access-list elements descriptive object-group names so that the connections's idientities are more obvious. I've always disliked how site-site VPN tunnel-groups need to be named after the remote peer IP.
In a previous position when I had occasion to do the same thing I also looked up the peer IP addresses in the whois databases (whois.arin.net and equivalents at RIPE and APNIC etc.). Sometimes that will point you to the remote partner identity
