Upvote:1
The airline will only check your documents relating to your current flight itinerary.
For example, with a return ticket from the US to Portugal, the airline will check that you can enter Portugal when you check in for your flight to Portugal. Your documents for entering the US will be checked when you check in for the return flight to the US.
As for entering the Schengen area, if you have a valid multiple entry Schengen visa and have used it before or it is issued by Portugal, you are unlikely to have problems with immigration.