If you browse a piece of JCL containing an EXEC PGM=<LOAD module> and the JCL contains neither STEPLIB nor JOBLIB, it gets a bit hard to find out which LOAD library <LOAD module> is located in. It is the same thing for any TSO command where the name of the command is equal to a LOAD module.
Fortunately it is very easy to find the name of the LOAD library where such a LOAD module is located. You start your search by entering ISRDDN (TSO ISRDDN does the trick). On the command line in ISRDDN you execute command LOAD <LOAD module>. The result is a pop-up panel which displays various information about the LOAD module including the LOAD library where it can be found. If no pop-up appears then you cannot execute the specified LOAD module without a STEPLIB/JOBLIB specification or in TSO an ISPLLIB specification or a corresponding LIBDEF.
In the pop-up panel it might be specified that the LOAD module will be loaded from LPA, PLPA or MLPA. In these cases you cannot see in which LOAD library the specified LOAD module is located. Then you may use the ISRDDN command LPA. This command creates a DD name on the list of DD names called LPA and displays the concatenation of datasets where LOAD modules located in LPA, PLPA or MLPA are fetched from. With the ISRDDN command M <LOAD-modul> LPA you are able to find the correct dataset provided you have read access to all the LOAD libraries on the LPA concatenation. It is rarely seen that we ordinary programmers have any access to those LOAD libraries, because the LOAD modules in the LPA concatenation are loaded into storage common to all address spaces at IPL. If you desperately need to know which LOAD library a LOAD module on the LPA concatenation is located in you have to contact the MVS systems programmers. They normally have at least read access to all of the datasets on the LPA concatenation.