Where can you find information about the value of old Valentines cards with honeycomb tissue paper fold outs?

Go to the Antique Road Show and they will tell you.

Also, go to Ebay and see what they are selling for. Once you come to the page with old/vintage valentines cards, click the "Completed Items" link on the left side of the page. It will tell you which ones sold recently and what they sold for.