In regards to knowing if he is ready, you would need to evaluate where he is at with certain commands while on-leash. Keep in mind the three D's: duration, distance and distractions, and work on one at a time. For example, if you are working on stays, I would ensure he is solid on all three D's on-leash before trying off-leash. Then when you first work off-leash, go "back to kindergarten" and work on one "D" at a time. You would start in an environment with no distractions, stand right in front of him and work up the duration of the stay before even thinking of introducing distance. When you start to work with distance, decrease the duration...etc.
I would pay special attention to his compliance with the "come" command while on-leash and work it in the house (or fenced yard) with him dragging a long line, making sure this is rock solid before venturing off-leash.
When you do go off-leash, use the long line for sure! I totally agree that a fenced-in dog park is best if possible, preferably with no other dogs there to start. Good luck!