Hi,
I am not sure the input voltage will connect to KL25 which module. If connect to GPIO input pin, you can check the Port Data Input Register (GPIOx_PDIR) about input voltage is high or low.
If you want to get the exactly value of input voltage, the ADC module could help. The ADC module will compare the input voltage with VREFH and VREFL voltage to get the input voltage value.
Wish it helps.
best regards,
Ma Hui