Dog Parks is really much like children's playgrounds. A few visits to the park/playground kinda teaches you how to identify problem dogs/kids. So that, when you see these guys, you'll be prepared to do what is necessary to protect your kids.
I love dog parks just like I love people parks. Some are better than others, that's for sure - usually because of the "clientele" that frequents the park. I've been to a dog beach that has poop all over the sand. Seems like nobody bothers to pick up after themselves even with the signs and plastic baggies and trash bins all over the place! And then I've been to a park that is completely poop free. Usually, the parks are somewhere in between - a few poop-leavers but usually only in the less-travelled areas. And interestingly, the amount of poop in a park also coincides with the behavior of the dogs in the park... only my own observation, of course, no scientific statistical data to back it up!