how to Return 1 true if exist SQL StoreProcedure.

One way to simplify this could be:

CREATE PROCEDURE Checkuser 
  @User varchar(34),
  @intOutput int OUTPUT
AS
BEGIN
   SET @intOutput = (SELECT COUNT(*) FROM aspnet_Users Where UserName = @User);
END
go

That should return the amount of users matching in the table. If the user name is unique, the result is 0 or 1.