I'd like to specify static DNS addresses using Google DNS, and use my ISPs DNS as the backup. Its straightforward to enter them into Tomato. But how can I determine my ISPs DNS address?
You can view "QOS-Details". Look for UDP connections on source port 53. Source address is the DNS server. Destination address is your WAN IP.
Check this just after browsing the internet or opening a couple of pages.
Specify Google DNS servers in the Static DNS fields.
Go to Advanced->DHCP/DNS page, and check "Use received DNS with user-entered DNS" box - that will make it to use your ISP's DNS servers as well.
P.S. If you want to send DNS queries to all servers at once, add "all-servers" line to "Dnsmasq Custom configuration" box. And if you want to use your ISPs servers only as a back-up in case Google DNS is not responding, add "strict-order" line instead.