Surely there's an easier way other than looking at every corner to find a gap.
Within an active sketch edit mode, go to Tools > Sketch Tools > Check Sketch for Feature. Pick any feature from the list which is relevant to sketch and then SW would find the issues for you (if any)
when editing the sketch,
select any segment and right click,
this will grab anything that is connected to that segment.
ideally the entire sketch will light up.
Retrieving data ...