Quote From TreeofLife:
Ask him for the reference but ask to see it rather than get him to send it directly. You might be surprised by what he puts.
You should always check with your referees anyway before putting them down as a reference.
This is spot on- I would also add that if he/she starts getting reluctant and stutter when you ask if you can see then you know it's not a good sign. Happened to me once.
If I were you put as many as possible and from the way they interact with you you'll get an impression who will write the best reference. Good luck!