Hi & welcome to the forum!
The low-speed fans should spin up when engine is on cusp of overheating. I forget what the trigger temp was on mine - somewhere around 114C.
Search this forum (General Technical Chat) for troubleshooting tips; the topic has been discussed ad nauseum.
Here are a couple to get you started:
2002 Focus Overheating.. Running out of Ideas
Fan sensors and wiring