On each host you can try running 'lsof | grep -i your_iso_file_name" or you can also try 'vmkfstools -D /path/to/file', and look at the vmkernel log for the following message:
Dec 17 15:45:38 YourServerName vmkernel: gen 56971, mode 1, owner 4909d046-a45dad72-a8b8-0017a48e5ab4 mtime 4151275]
Then you can run ifconfig on each host to see which host has the mac address above (0017a48e5ab4):
ifconfig -a | grep -i 5A:B4
vmnic0 Link encap:Ethernet HWaddr 00:17:A4:8E:5A:B4
Ben