Steve Hampton has a gull id website at
http://www.geocities.com/RainForest/Canopy/6181/gulls.html

Also it is always worth checking the ID Frontiers listsrev archives at
http://nbhc.com/birdmail.htm

You might also try Birding, vol 25(6), p. 386. The article is titled "Field
Separation of Ring-billed, Mew, Common & Kamchatka Gulls".

Cheers
Gail Spitler
Johnsons Landing, BC
spitler at wkpowerlink.com