This has been asked several times before, but I don't think there has ever been a good method.
You can turn on nfs.mountd.trace, but then you'd have to wait for a remount, and filter out all the other volumes.
You can turn on nfs.per_client_stats.enable, use nfsstat to list all NFS clients, and filter out hosts that appear elsewhere in the NFS exports, though this assumes there are no duplicates, which is unlikely.
Ideally, with that many hosts, there should be some sort of management (or at least shared ssh keys) that would let you check each one's mount tabs - though there you're assuming it's not dynamically mounted in a script or something.
Good luck!
Bill