How to know if a layer mask is full white ?

Here is another way

function selectLayerMask(visible){
    var rc = false;
    try {
        var desc = new ActionDescriptor();
        var ref = new ActionReference();
        ref.putEnumerated( charIDToTypeID( "Chnl" ), charIDToTypeID( "Chnl" ), charIDToTypeID( "Msk " ) );
        desc.putReference( charIDToTypeID( "null" ), ref );
        desc.putBoolean( charIDToTypeID( "MkVs" ), visible );
        executeAction( charIDToTypeID( "slct" ), desc, DialogModes.NO );
        rc = true;
    } catch (e) {}
    return rc;
};
function isRevealAllMask(){
    var ref = new ActionReference();
    ref.putEnumerated( charIDToTypeID( "Chnl" ), charIDToTypeID( "Ordn" ), charIDToTypeID( "Trgt" ) );
    var desc = executeActionGet(ref).getList(charIDToTypeID( "Hstg" ));
    var histogram =[];
    for(var h=1;h<256;h++){
        histogram.push(desc.getInteger(h));
    }
    return histogram[254] == histogram.toString().match(/[^,0][0-9]*/);
};
function selectComponentChannel() {
    try{
        var map = {}
        map[DocumentMode.GRAYSCALE] = charIDToTypeID('Blck');
        map[DocumentMode.RGB] = charIDToTypeID('RGB ');
        map[DocumentMode.CMYK] = charIDToTypeID('CMYK');
        map[DocumentMode.LAB] = charIDToTypeID('Lab ');
        var desc = new ActionDescriptor();
            var ref = new ActionReference();
            ref.putEnumerated( charIDToTypeID('Chnl'), charIDToTypeID('Chnl'), map[app.activeDocument.mode] );
        desc.putReference( charIDToTypeID('null'), ref );
        executeAction( charIDToTypeID('slct'), desc, DialogModes.NO );
    }catch(e){}
};
var hasMask = selectLayerMask(true);
if(hasMask){
    var revealAll = isRevealAllMask();
    selectComponentChannel();
    if(revealAll) alert('mask is all white');
}

Also if you have one of the newer versions of Photoshop that support mask density you might also want to check that. If the density is set to 0 this will not show the alert for non-all white mask even though the appear white in the GUI

Maybe someone has a better way than this?

toggleMask(true);
select();
activeDocument.selection.invert();
if(!hasSelection()) alert("this is a blank layer");
toggleMask();
function hasSelection (doc) { 
if(doc == undefined) doc = activeDocument;
  var res = false; 
  var as = doc.activeHistoryState; 
  doc.selection.deselect(); 
  if (as != doc.activeHistoryState) { 
    res = true; 
    doc.activeHistoryState = as; 
  } 
  return res; 
};
function toggleMask(Mask) {
if(Mask == undefined) Mask = false;   
var desc = new ActionDescriptor();
var ref = new ActionReference();
ref.putEnumerated( charIDToTypeID( "Chnl" ), charIDToTypeID( "Chnl" ), charIDToTypeID( "Msk " ) ); 
desc.putReference( charIDToTypeID('null'), ref );
desc.putBoolean( charIDToTypeID('MkVs'), Mask );
executeAction( charIDToTypeID('slct'), desc, DialogModes.NO );
};
function select() {
var desc64 = new ActionDescriptor();
var ref39 = new ActionReference();
ref39.putProperty( charIDToTypeID('Chnl'), charIDToTypeID('fsel') );
desc64.putReference( charIDToTypeID('null'), ref39 );
var desc65 = new ActionDescriptor();
desc65.putUnitDouble( charIDToTypeID('Hrzn'), charIDToTypeID('#Pxl'), 2.000000 );
desc65.putUnitDouble( charIDToTypeID('Vrtc'), charIDToTypeID('#Pxl'), 2.000000 );
desc64.putObject( charIDToTypeID('T   '), charIDToTypeID('Pnt '), desc65 );
desc64.putInteger( charIDToTypeID('Tlrn'), 1 );
executeAction( charIDToTypeID('setd'), desc64, DialogModes.NO );
};