I'm not sure it's even possible on a regular and reliable basis?
http://www.oceanwanderers.com/ONWillet.html
' Dennis Paulson writes: I guess it wouldn't be appropriate for me to transmit the (fairly brief) text on Willet subspecies in my new shorebird book until it's published by Princeton University Press, but I'll say that I've looked hard at the subspecies of Willets for a decade and a half. I had studied lots of them in several museums with large series when doing my 1993 book, and, while doing the new shorebird book, I examined recently collected and beautifully prepared series in the Burke Museum. I can only say they still leave me very puzzled. There are some great average differences, as everyone knows, but it is fairly common to find a specimen from the breeding grounds that could never be distinguished from its counterparts at the other end of the country - and this includes all the "classic" marks mentioned by Richard Chandler.'
Your bird does look pale and has fairly unbarred flanks FWIW but I don't think they'll be getting split anytime soon?
A