Most of the large (more than a few hundred meters) potentially hazardous asteroids are thought to have been found. However, there are some locations (relative to Earth's orbit) that are difficult to view from the ground, because you can only observe them at twilight. The best way to find those last remaining potentially hazardous asteroids would be to launch a space telescope to the Earth-Sun Lagrange Point 1.
In the grand scheme of things, this type of mission wouldn't cost that much, but so far, nobody has made the decision that making sure Earth doesn't get hit by an asteroid is worth an investment of a few hundred million dollars.