You didn't say which PIC24 so i have no idea of what solutions can be possible.
Anyway, if there is the AVDD/AVSS, AVDD mustn't differ more than 0.3V from VDD.
if you want to use an external VREF, VREF+ musn't exceed AVDD, VREF- mustnt go lower than VSS and VREF+ > VREF-
the PIC24 probably have a bandgap voltage reference available as an analog input so you can use it to measure Vdd as cv007 said, but you'll have to calibrate it because initial accuracy and temperature drift are not that good
otherwise, if you also have a different regulated rail or you are willining to put a reference voltage of any kind you can measure that one. i.e, a 2.5V one, measure it. a 5V one, use a resistor divider and measure it et cetera