To locate it with certainty:
1. Consult your owner's manual: This is the best resource for the precise location and a diagram.
2. Look near the coolant reservoir: The sensor is usually wired and fairly close to where the coolant level is visible.
3. Search online for a repair manual or diagram specific to your engine: Many websites provide free diagrams and repair guides. Searching for "2001 Chevy Monte Carlo [your engine size] coolant sensor location" will likely yield good results.
Caution: The engine will be hot if recently run. Allow it to cool completely before attempting to locate the sensor.