OP
you could maintain a CMOS or even NE555 oscillator (pray it will start up correctly but most likely these circuits always start up), and then, have a mono-flop together with an AND,
so you can hook a line from the PIC, and pulse this line with some 100 Hz (via software).
if this signal fails to appear (hence the monoflop) after a small while of maybe 100 to 200ms, you get the error condition!
if the signal disappears, you also get the error condition.
if you can ensure the uC starts up, just use the provided watchdog (but the circuit I describe also covers non-startup).
I hope you can understand it/build it.