The ecliptic is literally the line on which eclipses take place. However, that is a fairly rare occurrence, but the fact that eclipses, (as well as occultations and transits) take place there is what gives the line its name. In our solar system, the sun and planets are pretty much on the same plane, and this plane is called the ecliptic. The planets don't fall exactly into a plane, which is why the moon doesn't eclipse the sun everytime it is new. It is also why it is possible to see Mercury and Venus both before and after the sun has risen and set.
The theoretical description of the ecliptic being given, some simple observational tips should be given. And the short answer is, there is no way to easily detect where the ecliptic is. The only way to find it is to become familiar with the constellations that is passes through. These thirteen constellations are usually referred to as The Zodiac (consisting of the 12 familiar constellations, and also a bit of Ophiuchus. Some of these, such as Leo, are full of bright stars, and easy to find, while others, such as Capricorn, are dim and harder to locate without practice.
Someone wrote above that the ecliptic follows the plane of the galaxy, which does not seem to be exactly correct from my observations, although in some places, especially Sagittarius, they meet exactly. I must admit that trying to take the two dimensional sky, and picture the intersecting planes of the terrestrial equator, the ecliptic, and the galactic plane is a task that often strains my abilities. However, with enough practice the invisible lines across the sky begin to make more sense.