Based on what you have drawn
It seems that you want to measure the forward voltage, Vf of the LED segments. Since your LED segment cathodes are connected to ground you only need to measure the anode voltage with your ADC.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Measuring the forward voltage, Vf, of an LED.
Based on what you have written
I want to measure voltage presence when an led lights up (i.e. the ic_output goes LOW, led is common anode).
The -ve of led is connected using a 1k resistor to the output of an IC and +ve to VCC.
simulate this circuit
Figure 2. Measurement for a common anode LED.
This assumes that the LED anode is at the same voltage as the ADC Vmax.
- Turn the LED off by switching the GPIO high or disabling it.
- Measure the ADC voltage. This should be close to 1023 counts (on a 10-bit ADC).
- Turn the LED on by switching the GPIO low.
- Measure the ADC voltage.
- Subract reading 2 from reading 1. The result is the Vf. Convert that from counts to volts.