If I have ordered a travel from San Fransisco to Amsterdam, and American airline have given me tickets from San Fransisco to Denver and tickets from Denver to Amsterdam, can I then skip the first flight and check in at Denver instead?
The answer is NO.
If you miss any segment of an Itinerary, all following segments will be cancelled, in both directions.
For clarity, you will not necessarily lose any remaining value of the tickets, even if they are 'non-refundable'. You can apply that value to future travel, you just can't get the money back.