It depends on the dog park. We have a great dog park here -- people generally follow the rules -- dogs under voice command, clean-up, etc. However, if you are curious, I would say go visit the park first without your pup to check out how well behaved the dogs (and humans) are. Before taking your dog, I would also make sure Sadie is up on all her shots including those she needs if she is around other dogs (Bordetella, etc.). If you are going to let Sadie off leash, make sure she will come back. I might try taking her the first time on-leash. One more point of caution -- if Sadie isn't spayed -- don't take her -- for obvious reasons.
That said, Chloe LOVES our dog park. She is horrible on leash -- but just the best off-leash. She stays close, but can follow those yummy smells to her heart's content.