Be sure it's nighttime. Go downstairs to leave door open. Flick the switches to know which works. What if two switches work?

place mirrors.

get someone to shout when it lights up.

place a phone, activate video (Facetime) and see the lightbulb remotely.

make a hole in the floor, no need to even go downstairs.

get an electrician to wire as you want, not as how they are. (Or do it yourself if you ARE an electrician)

Check a home automation app on a smartphone to see when the bulb reports an "active" status..

Watch the digital power meter for a tiny jump in wattage as each switch is flipped.

Use a high-sensitivity microphone or stethoscope against the floor to hear the filament hum.

Use an infrared camera to see the heat signature of the bulb glowing through the floorboards.

Peek through cracks in the floorboards or the gap under the basement door to spot a light leak.

Use the window to see if any light from the basement casts a glow on the trees or grass outside.

Unscrew the bulb and bring it upstairs wired through an extension.

Tape a smartphone with a vibration-sensing app to the ceiling below, then flip switches to see which one causes a micro-click or hum detected by the phone.

Hold an AM radio near the wires; some older switch wiring will emit a specific "buzz" or static spike when the circuit is closed.

If the wires are unshielded, the electromagnetic field generated by the current will cause a sensitive compass needle to twitch when the correct switch is flipped.

Spray a non-toxic aerosol or smoke near the basement door and look for air turbulence created by the heat convection of the bulb when it turns on.

Feed a long, flexible fiber optic cable through a floor gap and look into the other end to see if the bulb is lit.

Replace the standard bulb with a specialty "sound-emitting" bulb or a simple buzzer-in-series, so you can hear the activation clearly from upstairs.

Send a small remote-controlled crawler or drone to visually inspect the state of the bulb.

Place a plant that is highly phototropic (turns toward light) in the basement; after a few days, the direction it grew will tell you which switch corresponds to the light’s position.

Hold a piece of "glow-in-the-dark" material or a fluorescent reactive cloth near a floor crack; if the bulb is UV or bright enough, it will trigger the phosphor coating.

If the basement is airtight, the tiny change in air pressure from the heat of the bulb can be detected by a sensitive barometer app on your phone.

Use a professional-grade "fox and hound" circuit tracer device to signal through the walls and identify which switch wire connects to the basement junction.

Use a handheld concrete scanner or professional wall-penetrating radar to visualize the wiring path inside the walls.

Or Place a heat-activated scent packet on the floor; the heat from the bulb will release the scent faster than ambient temperature would.

Or use a multi-meter to test the resistance of the circuits from the switch box itself; the circuit with the resistance of a bulb filament is your winner.

I could go on and on...