How to find status (on/off) of any applicance ( light, fan, etc.) connected to an electrical circuit
Figure 1. CR-Magnetics CR2550 current indicator.


  The CR2550 series Remote Current Indicators are a low cost method for providing a visual indication of electrical current flow. The indicators are factory calibrated to provide a preset turn-on point. The value of the turn-on point is determined by the customer and specified in the part number. Attached to the transformer is a high efficiency, bi-polar LED that illuminates when the current is above the turn-on point. The indicator is available as standard with 11 inch long leads and a red LED indicator. designed as a low cost method for providing a visual indication of electrical current flow. The current-carrying wire is routed through the window opening in the current sensing transformer. Attached to the transformer is a high efficiency, bi-polar LED that illuminates when the current is above the turn-on point. The indicator is available as standard with an 11 inch long lead and a red or green LED indicator.



Turn-on Point: 0.75 Aac RMS
Maximum Continuous Rating: 20 Aac RMS
Frequency: 50 to 400 Hz
Operating Temperature Range: -30 C to +60 C
Storage Temperature Range: -55 C to +85 C
LED Indicator Type: Bi-polar, Red/Red or Green/Green
LED Mounting Hardware: Plastic one-piece, press-in lens supplied


From the spec you can see that for this model you would need to get the ampere-turns through the coil to between 0.75 and 20 A when your load is on. For example, with a 0.25 A load you would need four or five turns of your current carrying conductor through the transformer to ensure reliable switch-on.

There are a few application notes on the linked page.

You can glue the LED to an LDR to signal to your micro-controller or remove the LED and replace it with the input of an opto-coupler. 



Figure 2. The CR-Magnetics devices use LEDs as shown on the right. If converting to an opto-isolator it would need the same LED arrangement internally.

Note that the spec says the LEDs are bi-polar which means that there are two back to back in the one package for AC.