When a Prebid.js package is built, the list of modules compiled into it are placed in the pbjs.installedModules array.
e.g. if this builds the package:
gulp build --modules=a,b,c
pbjs.installedModules would have the value [‘a’,’b’,’c’].
If you happen to compile in all 400+ modules (not a good idea!), the value of pbjs.installedModules will be an empty array.