There are a few things you need to know about differences between chemtrails and contrails. I remember in my childhood watching passing jet planes and seeing their traces disappearing within seconds. We see the same pattern today as well. However, in addition to that old behavior, there is the new kind of trails; the kind that do not disappear so quickly...

All the photographs below were taken on 9 May 2004 not far from the Viejas Casino, off the Highway I-8, looking towards the Los Angeles Basin.

This first image [above] was taken from a lower location. You can actually see where the spraying was cut off - at the beginning of a turn. Except for the two trails that continue across the sky, overhead, all the trails (mid-low section of the photo) die and turn.

The above picture was taken from a more elevated location. This is a little zoom-in into the trails. You can see the widening trails in the evening light. 

Above: a wider area, where more trails are visible. 

Another one, showing more of the thickening trails.

Below: a sequence of pictures showing a chemtrail in progress...

		You can see the plane changing direction, at the top of the trail, and turning off the nozzle.
		Heading to the left, upward. Only regular contrail visible. Will disappear in a few seconds (see the next picture).
		Regular contrail disappeared; only the chemtrail still visible...
		Notice the bottom of all the images; the chemtrail continues to get wider.
		The bottom is getting thicker, and becoming like the other trails you've seen on the bigger pictures.

One has to wonder why do they have to turn off the spraying at certain point. Except for the two streaks heading overhead, all the other ones would turn their nozzles off and change direction. There is no need to speculate - just observe what's happening. If you ever see jets passing overhead, leaving trails that don't disappear within seconds, you are most likely witnessing spraying. It occurs mostly over heavily-populated areas.

There are literally thousands of photographs of chemtrails you can find on the internet. Probably one of the biggest depositories of chemtrail images is Chemtrail Central.[2] 
You can often find links to freshly-posted images on websites like Propaganda Matrix, InfoWars.com, The Power Hour, and elsewhere.[3] Be aware, when doing your own research on the Internet, that there is a lot of disinformation planted there as well.

1. Find out more about chemtrails - http://www.carnicom.com/contrails.htm
2. See some spectacular chemtrails - Chemtrailcentral.com
3. URLs: http://www.propagandamatrix.com - http://www.infowars.com - http://www.thepowerhour.com