Usually either L+R+Start+Select, or Start+Select. But most of the time, you have to set it.
From RetroArch Menu (Load Core, Load Content...) - Press Right to go to the Gear Listings (User Interface, Video, Audio...)
Go down to INPUT
Go down to HOTKEYS
Go to MENU TOGGLE
Press Right until you see a Combo you want.
Press X (Default), or O twice to go back to the main Gear Listings again
Press Left to go to the Main Listings.
Go down till you see CONFIGURATION FILE
Press X (Default) to go into it
Go to SAVE CURRENT CONFIGURATION and best to also SAVE NEW CONFIGURATION just in case, and press X (Default)
Press the HOME Button and Exit RetroArch.
You should now be able to get the Menu to Pop up with the Button Combo you Set