First step would be to start with engagement surveys and then analyse the results.
Different people are motivated in different ways, so there may not be a one size fits all solution; but teams tend to believe similarly. And survey results work best when directed for a team as a whole.
If you frame your surveys with the correct context and ask nuanced questions, the results will give you a fair indication of whats working and whats not.
For retention programs, the first step is to identify there is an attrition risk - predictive analytics help here greatly.
For every employee, taking ONA interaction frequency, engagement scores, pms scores, increment history and internal + external compa together, gives a good insight on flight risk.
Once identified, these folks can be proactively put on a retention plan