Unless the EAP process was started using systemd, there is no PID file, just a java process running the jboss-modules.jar JAR file.
So you can find out what your EAP instance's process is using any of the ps utils (ps, pgrep, etc.), for example:
$ pgrep -f jboss-modules.jar
Note that if you have multiple instances running, you will see multiple PIDs in the output, and to be able to better distinguish them, it may make sense to add the "-l" option, which will also display the command line used to start the instance: