You can check it by the PGM=AMBLIST. If the POPEN is included in a user module, the PAM is statically linked. Please execute it and check the module including the POPEN as CSECT.
//AMBLIST EXEC PGM=AMBLIST
//SYSPRINT DD SYSOUT=*
//SYSLIB DD DSN=Prefix.user.loadlib,DISP=SHR
//SYSIN DD *