Using this function (constructor):
func NewDecoder(r io.Reader) *json.Decoder
How can you identify who is a io.Reader in the standard library? I know that a os.Stdin is one, but how do you find other readers in the standard lib?
Short of searching the GoDocs for io.Reader (https://golang.org/search?q=io.Reader) or using Google to do something similar you'd have to write a program to parse the Go Standard Library.
Perhaps a more useful avenue of inquiry would be, telling us what you're trying to accomplish.