The 16F690 has an internal 0.6V reference that can be turned ON/OFF from the VRCON register.

With VREF+ set to VDD, you can use the 0.6V reference to measure the PIC's VDD voltage. Which goes something like this ...
Code:
VDD   VAR WORD

VRCON.4 = 1        ; Turn 0.6V reference ON
PAUSEUS 100        ; Allow VP6 to settle
ADCIN 15, VDD      ; get VP6 analog reading (10-bit)
VRCON.4 = 0        ; Turn 0.6V reference OFF
VDD = 6138 / VDD   ; convert to voltage
The variable VDD now holds the VDD voltage *10 (3.3V = 33).
The formula is the same for any VDD voltage.

Then ...
Code:
IF VDD < 32 THEN GOSUB LowVoltage   ; if less than 3.2V
This then tells you when the battery voltage has gone below the LDO's "Drop-Out".
Or you can set it to any other voltage (lower than normal VDD) as required.

hth,