As for how individual spells look, I have always gone with the school/organization/mentor/etc.. method. Each of these does it a little bit different (nothing to change the rules though) and would have minor appearance changes to them, as well. So, the royal-mages of the Kingdom Kunia have Shield spells that look like a white, kite shield with the emblem of Kunia. The Shield spell of the street mage, on the other hand, looks like a silvery round shield with no emblem. Once it is established, the only way to change it is through Spell Artistry (or Mastery, if you prefer).
Not every spell has such distinctive characteristics, I believe. Some could be completely unnoticeable, unless you have the required magical abilities or spells.