How can I tell which Instagram filter was used?
you can actually see which instagram filter people use in they photos. Just use a web browser for instagram. That filter is called "Walden" as you can see here: http://square-pics.com/m/858325353241497438_1085312802.
There are many other browsers, just google them.
Hope it helps :)
Edit: By "web browser for instagram" i mean an external website designed to navigate through Instagram photos and profiles, another way to use instagram with a non-official web. Instagram.com does not allow you to have a "most popular photos" screen when you log in, plus doesn't show you the filter used in the photos, tbh, official not-mobile version of instagram is really bad and archaic. Websites designed to browse instagram like square-pics work with Instagram's API and add aditional features (have a nicer design, shows filters used, popular photos and users, a nicer view and preview of the photos, possibility to see comments, likes and descriptions without having to enter to photos...) that you can't find in the official Instagram website for PC.
That's the reason why you can find a lot of this kind of websites.
square-pics is down : http://downoruprightnow.com/status/square-pics.com