What happens during a cleaning at the dentist's office?

I haven't been to the dentist for a long time and it took a major tooth ache for me to go. But that was just an emergency appointment and the dentist only checked that little area. I booked a cleaning for next week and will be under the care of a dental hygienist. During that time, will she tell me if I have cavities? Or is that only during a proper check up with the dentist? And is it possible that the hygienist will stop cleaning before all my teeth are done and tell me to book another appointment cus it's so dirty?