Possible trouble in paradise: how to detect PoE ready?

Added 3 resistors to pull the ADC in appropriate directions: towards +5,
towards ground, and towards the Ethernet leg.  The last will be either:
+48V: PoE is sending power, ADC gets 4.36V plus a little pull-up from
the +5V resistor.  As long as it's not more than 5V, whatever.
25k resistance to ground (PoE ready but not on yet): ADC gets 1.17V.
Open circuit (Not PoE ready): ADC gets 4.78V.