Here's the promotional video from Rafael, the system's maker. If the Iron Dome launchers are in a position to hit incoming rockets when they're still in boost phase, they're clearly effective. When they hit, the ascending rocket's flare disappears. Israel has Iron Dome launchers both forward postioned near Gaza, for boost phase defense, and near cities, for terminal defense. For terminal defense, it's harder to tell if they worked. The incoming rockets are just falling at that point, and success requires blowing up their warhead, not their rocket engine.
Videos show the missile's warhead exploding. That's triggered by a proximity fuse. There's a spray of shrapnel from the warhead; it doesn't have to be a direct hit. Whether that sets off the incoming rocket's warhead isn't visible from the videos of terminal defense.The Patriot missiles used in the Gulf war were able to hit incoming Scud missiles, but often didn't detonate the warhead.