Any field that looks reasonably "clean", is easily reachable from my present altitude and is long enough, preferably into the wind, is OK.
Don't go through a long winded decision making process but trust your instincts. And unless the field turns out to be completely hopeless, stick with your decision. A good landing in a bad field is more survivable than a bad landing in a good field.
Also remember that as soon as the engine quits (for real), the plane is no longer yours but belongs to the insurance company. The objective is to be able to walk away, not to save the plane.