Code:
procedure Validate(EnteredText : string);
var Valid : boolean;
     i : integer;

begin
  Valid := true;
  for i := 1 to length(EnteredText) do
    if EnteredText[i] in ['0'..'9'] then
      Valid := false;
  if not Valid then  
  begin
    // do whatever you want
  end;
end;
Did not test, just wrote it out of my head in a hurry. Maybe you have to alter some things in " if EnteredText[i] in ['0'..'9'] then" but thats how you could look for digits in the string.