<ul class="slides">
<li>
  <img src="slide1.jpg" />
  <map id="map-one"></map>
</li>
<li>
  <img src="slide2.jpg" />
  <map id="map-two"></map>
</li>
<li>
  <img src="slide3.jpg" />
  <map id="map-thre"></map>
</li>

<li>
  <img src="slide1.jpg" usemap="#map-one" />
  <map id="map-one"></map>
</li>

var $mapid;

$('.slides li map').each(function(i, elem) {
  var $this = $(this);
  $mapid = $this.attr('id');
});

console.log($mapid);

<ul class="slides">
<li>
  <img src="slide1.jpg" usemap="#map-one" />
  <map id="map-one"></map>
</li>
<li>
  <img src="slide2.jpg" usemap="#map-two" />
  <map id="map-two"></map>
</li>
<li>
  <img src="slide3.jpg" usemap="#map-thre" />
  <map id="map-thre"></map>
</li>