If the option isn't visible, click on the game area, then on the top left "☰" icon. It should show the emulator's UI with icons for fullscreen, mute/unmute, inputs, and the emulator's help screen. The inputs icon opens a screen where you can see and/or change all of the input settings.
If I recall correctly, the default inputs are:
Player 1:
* Up/down/left/right = up/down/left/right
* F = A
* D = B
* Enter = Start
* S = Select
Player 2:
* I/K/J/L = up/down/left/right
* Q = Start
* W = Select
* R = A
* E = B