FormatValues

FormatValues[f] gives a list of transformation rules corresponding to all printing formats (values for Format[f[x, ...], ...], etc.) defined for the symbol f.

Examples

Define a format for a symbol:

Format[mySymbol[x_]] := Framed[x]
FormatValues[mySymbol]

Check format values for a built-in symbol:

FormatValues[Plus]

Please visit the official Wolfram Language Reference for more details.