2V5 is in the analog world so if you have a spare comparator or ADC pin on your PIC then use it.

If you do not have a spare analog pin, then RCTIME and POT commands can be used.

Setup your PIC with a fixed resistor and capacitor for RCTIME or POT on an available digital I/O pin. Connect the 2V5 source to the same pin through, say, a 10K resistor. There will be large changes in the value returned by the RCTIME or POT command when the 2V5 source is present/not present.

HTH
BrianT