The tooltips for the actions will include the information on combo effects. For example, the tooltip for Skull Sunder tells you:

Combo Action: Heavy Swing
Combo Potency: 200

That tells you that using Heavy Swing first will turn Skull Sunder into a combo, with increased potency as the combo effect.

Some classes have it a little different. PGL/MNK, for example, has abilities that change the fighting stance/form they're in, and abilities that can only be used from one specific form. They get the dotted lines for abilities that are usable in their current form.

As an Archer, you have things like Heavy Shot. The tooltip tells you there's a 20% chance that your next Straight Shot will deal critical damage (if you use it within the duration, 10s). That's what causes the dotted lines around Straight Shot.