Code P0442 on 98 4.3

Hello,
Yes I did do a search on this subject and thanks to the efforts of Blazee and others it was very informative.My question is how does the PCM know that the fuel cap is off or sense other evaporative system problem.
Thanks
Unbe

Periodically the pcm actuates a vacuum system that pull down the entire evap system to see if it will hold vacuum , if it detects the systems inability to hold this vacuum it sets a code.

Maxwedge
Thanks for the info.
Unbe