In the help text popup for a skill you should see a combo action listed, which is the lead-in for a combo. I.e. Savage Blade lists Fast Blade as a combo action, so performing Fast Blade followed by Savage Blade results in a combo.

This dotted line highlight on skills also occurs when a random-chance effect is triggered. The Archer trait Heavier Shot has a chance to cause Straight Shot to deal critical damage, and when this chance occurs, Straight Shot is highlighted. This does not imply that these two are combo actions, only that one has a limited time effect on the other.