However for me, one thing I really like doing is reaching out to employees who quit/left. Ask them for honest information about why they quit and what the existing problems are.
The baseline approval rating from someone who left the company is going to be lower than people working at the company. If an ex-employee is disgruntled or bitter it doesn't tell me that much, unless I hear the same complaint from a lot of people.
If someone left because another great opportunity came along, and they are really positive about their time at the company, I'd say that's a good signal.