//variable to evaluate if radio button is checked
var optionPanel = false;
//loop to check if the user checked the radio button
for(var x = 0; x < document.formLogin.select_Panel.length; x++)
{
     if (document.formLogin.select_Panel[x].checked = true)
     { 
          optionPanel = true;
          //code what must be done here if one of the radio buttons are checked
     }
}
 
//Code if the radio button is unchecked or unselected
if (optionPanel = false)
{
    alert("PLease select one of my radio buttons");
    return false;
 } else {
     alert("Form values are valid");
     return true;
 }