Check the lighting on both sides
Tap the glass and listen for a hollow sound
Press a fingernail against the surface and look for a gap
Turn off the lights in your room and see if the other side is visible
Shine a flashlight at an angle and look for reflections
Look for a mirror-like surface on one side and clear glass on the other
Ask someone to stand on the opposite side and see if you can spot them
Use the phone camera flash to check for transparency
Look for signs of a hidden room or observation area
Contact building staff or the property owner for confirmation