This is more a comment than an answer, but since I have a picture, I write it as answer.
It has alredy been written that contrails may form aside or behind an aircraft, so you may or may not be able to see them from the rearmost seats.
But also note that the blast stream can be much lower than what you would expect. In a two-engine aircraft, you have to look down from the upper edge of the window, and even then, the stream will be on the lower border of your field of view. Of course, the stream of the outer engines of four-engine aircrafts will be more centered in your field of view.
If you are flying over a scenery with high optical contrast, you can recognize the blast as a sharply defined region where the view is very blurry, as in the lower right region on this picture:
This was taken from the last seat row of a A340 after takeoff at Chicago O'Hare, and you can see the blast of the outer engine. (And yes, the outer stabilizer is in a little weird condition...)
So, look out for these blurry regions, that's where contrails may form.
EDIT: (just for fun)
Coming home from vacation yesterday, I noticed another sign for contrails from your aircraft: The contrail's shadow:
Unfortunately, the clouds became less dense when I made the photo, before I could also see a good shadow of the aircraft itself.