From type/Parameter

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method suffix(Parameter:D: --> Str:D)

Returns the ? or ! marker a parameter was declared with, if any. Otherwise, returns the empty string.

my Signature $pos-sig = :($a, $b?);
say $pos-sig.params[0].suffix; # OUTPUT: «␤» 
say $pos-sig.params[1].suffix; # OUTPUT: «?␤» 
 
my Signature $named-sig = :(:$a!, :$b);
say $named-sig.params[0].suffix; # OUTPUT: «!␤» 
say $named-sig.params[1].suffix; # OUTPUT: «␤»