There are several problems here:
Power-theft in this manner is not feasible: Even if you had a couple kilometers of wire paralleling the power lines, you would only get a few watts of energy.
Furthermore, the inverse-square law comes into play. Mythbusters tested this, and cot a few milliwatts with a great big wire coil right up against the wires. This would be rather noticeable, however. The available energy at the base of the tower is much less.
Note: It's possible to harvest a bit more power from the fields of a power line if you have a circuit that wraps completely around the power line (therefore basically acting like a 1-turn transformer). However, you are stuck with capacitive coupling if you cannot wrap entirely around the wire. (This is how parasitic power monitors work, like the one AngryEE mentions. Also, most parasitic monitors use microwatts of power anyways, so they don't need much power to work.)
Basically, the only real way the power company can find you did something like what is described in the question is to physically observe it. Realistically, if you did what you describe, the company would pretty much come after you for trespass and (maybe) vandalization, not theft.
Interestingly enough, on some power lines, there is no way to tap the line at all without a physical connection - On some long distance transmission power lines, they use high-voltage DC to reduce losses. On those power lines, there is no way to capacitively or inductively couple power out of the wires.