It is common to say "Foo implements the Fooer interface" in the doc on the function. Also, looking up what interfaces a type implements is 1.) impossible (depends on what other interfaces exist in the universe), 2.) pointless - you shouldn't care what interfaces it implements, you should look for functions that you want to use with your type.
You're also missing one of the best parts of implicit interfaces - you can use other people's types in your own functions with interfaces that didn't even exist when they wrote their type. To do that in languages with explicit interfaces, you have to write ugly wrapper classes around the other person's type for zero benefit.
