How do I validate that the values are not left blank? One is a simple form value and the other is generated by a calculation.

Code:
//I need to validate that these form values are not blank prior to submission of the form
                                <form id="sendit" action="http://ntsacom.homestead.com/TEST3.html" method="get">
                                Player Name: <input name="PLAYER_NAME" />
                                <br/>
                                <br/>
                                                            <div id="age" style="visibility: hidden;">
                                                            League Age:<input name="league_age" readonly />
                                                            </div>
                                                            <input type="submit" value="Submit this player" />