FormatValues[symbol]First, use Format to set a formatting rule for a form:
Format[F[x_], OutputForm]:= Subscript[x, F]
Now, to see the rules, we can use FormatValues:
FormatValues[F]
The replacment pattern on the right in the delayed rule is formatted according to the top-level form. To see the rule input, we can use InputForm:
FormatValues[F] //InputForm