OMG... this isn't going to become a training course in video for the hobbyist...

Here's THREE solutions...

1. Detect the Video as a whole. In the same way you would strip the audio modulation component from a Radio Carrier, you can strip the video component. Couple of Capacitors, a Resistor and a Diode. Noise (when a carrier is not present can simply be filtered away), and when video is present, you simply look for a decent voltage from your Detector.

2. Detect the Frame Pulses. The Frame Pulse is the start of each picture. The repetition rate is slow enough even for a handicapped (frequency challenged 10F PIC) to cope with. However, different cameras and standards mean that the Frame Rate could be as slow as 5fps or as fast as 120fps. Still, you're looking for a repetitive, slow pulse...

3. Detect the Line Sync. Build a sync-stripper and detect the LINE sync (my choice). You can build a sync stripper with not much more than one Transistor. The presence of LINE sync means you've definately got sensible video (in some cases Frame Pulses on their own with no other video information are used to keep a carrier link open, and since in some cases the Frame is phase-locked to the supply mains you have to make sure you're not simply detecting supply mains hum).

Now, armed with all these choices you can go and learn something about video detection and sync stripping...