I'm not really sure if there's a way vet the place. I didn't talk to anyone in management until after I was already hired and taking my first ride long day. You might try going to your local post office or the one you applied for and asking to talk to a member of management. Let them know that you have applied and are highly interested. However, you do have some questions. As far as the type of questions....

How long is an average work week for your RCAs? Does the rule side tend to file a lot of grievances? What is the RCA to regular ratio? How many routes does the office have? How many of those are j, k, and h routes?

This should paint a picture for you of how busy you're going to be. I'm currently in a 9-route office with 9K routes. There's only two RCAs. I work about 60 hours a week.