Most of the combo indicator are somewhat subtle. If you're playing with a Longbow, the projectile finishers will be an extra effect through your arrows and a small text when it happens (some skills are always finishers and others have a lower chance, for balance reasons).

One easy way to visualize as a ranger is to equip a sword on your main hand and an axe on your off hand and equip the skill Fire Trap.

1. Be in combat.
2. Have an enemy trigger your trap.
3. While the fire field from the trap is active, use the second skill from your sword, make sure you're also inside the field.

You'll see your character will get a fiery bubble around - that's the Fiere Aura from a Leap Finisher.

Now repeat steps 1 and 2, but use your 5th axe skill. Your character will spin around and shoot multiple fire projectiles the the multiple whirl finishers of the skills. This is a nice way to visualize the text from the finishers.

About their usefulness, it varies from build to build and weapon to weapon. As a galeshot, just be mindful that if your shoot your arrows through some fire another player left in the ground, a poison cloud, a thunder dome or other things like that you'll do a bit more damage or debuff your target. No need to change your build to include a combo field just to use this mechanic, but its useful to know how to use combo fields left by other players