Barbara - Blue Dasher is smaller, has a dark thorax with complicated frontal and lateral markings and has a pale blue abdomen with a black tip. Also,this species lacks the yellow coloration on the frontal wing margins possessed by Yellow-sided Skimmer. This common name by the way is more attributable to the juvenile form which has very obvious yellow lateral coloration. This becomes largely obscured by pale pruinosity on mature adults. Also look at the eyes. Dark gray/blue on Yellow-sided versus bright grren/blue on the Blue Dasher. Other differences but I have rambled too long. Hope this helps. - Allen Barlow