Use IsNaN to test whether a variable is set to a valid number versus N/A:
def myvar1 = 5;
AddLabel(yes, myvar1 + " is a number", if IsNaN(myvar1) then Color.RED else Color.GREEN);
def myvar2 = if close > 3000000 then close else double.nan;
AddLabel(yes, myvar2 + " is not a number", if IsNaN(myvar2) then Color.RED else Color.GREEN);