If you run brew services it should show the path to the plist file that brew uses to run the service. For my machine this points to ~/Library/LaunchAgents/homebrew.mxcl.mongodb-community.plist. This file will contain, among other things, the program arguments which controls how brew would start things up:
I see that I made a typo and the config file is mongod.conf and not mongodb.conf. Sorry about that.