Another good thing that's been totally screwed up.
Your instinct to ask first is the right one. If you had just shown up you'd probably have received unsympathetic attitude from the ticket agent and/or TSA detail at the metal detector.
In the old days, a holder of a foreign pilot certificate probably could've just chatted with the Capt for a bit and if he was in a good mood then bingo, you got the jumpseat.
Today, after 9/11, and bearing in mind certain security requirements as I respond to an anonymous post on the internet, I'll just answer your question like this: You need to be employed by an American company that participates in a new program for jumpseaters that positively identifies you through a computer database. If you're not listed in that database then you're not getting the jumpseat.
That's the short answer.