Detect when A/D occurs.

[JimB]

Is there a way to monitor when exactly an A/D conversion starts or ends?

[Ttelmah]

An A/D conversion, starts when you tell it to start. It finishes when the ADIF interrupt bit gets set. The default code (read_adc()), starts the conversion when you call it, and does not return till this flag gets set.
If you want to do other things during the conversion, define ADIF yourself, with the #bit statement, and then use 'read_adc(ADC_START_ONLY)', which will start the conversion, and not wait for the completion, then just monitor 'ADIF', and when it goes true, the conversion has finished (remember to clear it yourself, before using it again).

Best Wishes