I have one of these neon-lamp, internally illuminated, switches configured as a SPDT two-way switch to control an upstairs light from a position on the upstairs wall. It's companion, an ordinary SPDT two-way switch without the neon lamp, is mounted on the downstairs wall near the foot of the stairs. The upstairs lamp only illuminates if the light it controls is off, but that's okay as the switch lamp is only needed to find the switch on the wall when the room is dark. Perhaps you could replace your existing fan switch with one of these neon-lamp illuminated switches, but the switch neon-lamp would only be lit if the fan is
off because the lamp would find its return through the fan motor.
Ok. You might have to dumb it down a little for me.
The hot side of the 115 V AC power line goes to one pole of your existing SPST fan switch. The other pole of the SPST switch goes to one terminal of your exhaust fan motor. The other terminal of the fan motor connects to neutral (somehow) but that is not important. The fact that it
does connect to neutral
is important, but that doesn't have to occur inside the box at the fan switch... and probably doesn't.
If a small neon-lamp with a current-limiting resistor is placed inside a translucent switch handle and internally wired across the two switch terminals (by the switch manufacturer), and a load (such as your fan motor) is presented on the switched circuit side, then when the switch is closed the load is energized (exhaust fan runs) and the light will be
off, because the switch shorts out the light while allowing power to pass to the load. When the switch is open, the neon lamp and its current-limiting resistor are placed in series with the hot power line and the exhaust fan motor. This is a high-impedance connection, so the fan motor doesn't run. But the fan motor impedance is low enough to allow the neon-lamp and its current-limiting resistor to draw enough current to illuminate the lamp.
Of course this is the
exact opposite of what you asked for, but it
does provide an indication of the status of the exhaust fan: light on, fan off; light off; fan on.
And all you have to do is replace a switch.
There are also switches made with a separate toggle switch and neon-lamp The current-limiting resistor is usually built-in to the lamp assembly. and this combination will work as you originally requested
only if there is a neutral wire present in the original switch box. The lamp is internally wired between the load (exhaust fan) terminal on the SPST switch and the neutral wire terminal provided on the switch case..
So, you first need to determine is if there
is a neutral (white) wire in the existing box. DON'T take the white color of a wire in a switch box as being gospel! If there is only a black wire, a white wire, and possibly an un-insulated or green wire in the switch box, NONE of these wires is connected to neutral. Always test to make sure any white wire you find is really a neutral wire. The electrician who connected the switch is supposed to use black paint to identify any wire that could be "hot" but when wiring a switch to a load, but they often don't bother.
Today, the National Electrical Code requires safety (third wire or green wire) grounds everywhere, bonded to metal handy-boxes if that is what is used to house the switch or a receptacle, and continuous back to the distribution panel in any case. I don't think the following is kosher according to Code, and therefore I cannot recommend it, but you
could use that safety ground in place of a white neutral wire to provide a return path for the neon lamp. Problem is, that connection could (probably will) trip a GFI circuit breaker when the exhaust fan is turned on. The better alternative is to snake a white neutral wire from the switch box through the wall(s) and connect it to the neutral bar in the circuit-breaker power-distribution panel.