IsClassFunction( obj )
returns true if obj is a class function, and false otherwise.
    gap> chi:= S4.charTable.irreducibles[3];
    [ 2, 2, -1, 0, 0 ]
    gap> IsClassFunction( chi );
    false
    gap> irr[3];
    Character( S4, [ 2, 2, -1, 0, 0 ] )
    gap> IsClassFunction( irr[3] );
    true 
GAP 3.4.4