As a matter of curiosity, do many pilots position/jumpseat with airlines other than their own on aregular basis, and what checks are made?
Only in the movies, not allowed in real life.
Wrong..... It is allowed in real life. Many airlines still allow pilots from other airlines to "jumpseat". Captain does a quick check of their security pass and ID, and they sit in the passenger cabin on a space available basis.
Many commuting pilots depend on this to get to work.