This reference will help you know what the various game controllers look like if necessary.  All of the classic games in retroarch can be played with a SNES controller.  Most of the older systems used few buttons in general. You can also use an Xbox360 controller which will map to the same layout as the SNES without any work.  Just keep in mind the positions of ABXY on the Xbox Controllers is backwards to SNES and will not correspond by letter.  So Learning the SNES layout is important to correspond to other controllers gameplay.

SNES

ATARI

GAMEBOY

GAMEBOY ADVANCE

SEGA MASTER SYSTEM

SEGA GAMEGEAR

SEGA GENESIS

SEGA DREAMCAST

NINTENDO 64

XBOX 360

PLAYSTATION