I would populate your IsInternal property as a "data" attribute in each element so you can read it back easily in jQuery:
<input type="checkbox" name="jobOffers" class="chk-jobOffer" data-is-internal="true" value="1">Offer 1
<input type="checkbox" name="jobOffers" class="chk-jobOffer" data-is-internal="false" value="2">Offer 2
<input type="checkbox" name="jobOffers" class="chk-jobOffer" data-is-internal="false" value="3">Offer 3
<input type="checkbox" name="jobOffers" class="chk-jobOffer" data-is-internal="true" value="4">Offer 4
</input>
Then, in jQuery, check the data attribute before showing the alert (remember that jQuery transforms the data attributes into camel-case values):
$(document).on('click', '.chk-jobOffer', function() {
if ($(this).is(':checked') && $(this).data('isInternal') == 'true') {
alert($('#PostErrorMsg').val(), $('#PostMsgTitle').val());
}
});