1. Does she hear me well now?
2. Can she hear me well now?
The difference is minimal; you could ask either question whether 'she' is present or not.
3. We saw a lot of people there.
4. We could see a lot of people there.
No. 3 is a normal comment in any circumstance. No.4 carries the suggestion that maybe it was not easy to see a lot of people but nevertheless we could see a lot, or, referring to a coming event: (maybe) we could see a lot.