Turn your volume down first, then press Left, Right or Both to play a short test tone. Confirm the sound comes from the expected side and stop the tone at any time.
A steady tone from only the channel you selected. If the left button produces sound on the right, your cable or system balance is swapped; if one side is silent, that driver, cable or output channel has a fault.
The tool caps its output gain well below full scale and ramps the tone in and out rather than starting abruptly. Still, start with your system volume low and raise it gradually, especially on headphones.
Browsers play through whatever output your operating system has selected, and only some expose per-page device switching. Change the default output in your system sound settings if the test plays through the wrong device.