A car making a left turn is almost always liable to a car coming straight in the other direction. Exceptions to this near-automatic liability can occur if:
Whatever the contributing factors, the law says the car making the left turn must wait until it can safely complete the turn before moving in front of oncoming traffic. Also, the location of the damage on the cars sometimes makes it difficult for the other driver to argue that the accident happened in some way other than during a left turn. So, if you have had an accident in which you ran into someone who was making a left turn in front of you, almost all other considerations of fault go out the window, and the other driver is nearly always liable.
For more information on current traffic laws and court procedures, see Beat Your Ticket: Go to Court & Win, by David W. Brown (Nolo).