判断查询结果是否为空

位大哥帮个忙，string sqlstring = "select * from admin where usernumber='" + this.usernumber.Text.Trim() + "' and userPwd='" + this.pwd.Text.Trim() + "'";
        loginSDS.SelectCommand = sqlstring;
我想判断查询结果是否为空？怎么做？