a few things I do is look at structures. Where would people have congregated there? (ex. is there an old bandshell or bandstand? old WPA picnic benches?) Next, I like to look at the ground by the trees. Are there roots showing nearby? If not, a good sign of fill dirt which makes things more difficult.
Another thing I do is to use wheat pennies as a gauge. If I'm not digging one up at minimum every 30 minutes, I'm not in the right place. I actually prefer them more frequently. I figure if the wheaties are not there, the silver won't be there either in most cases.
This of course is not an exact science. I had a trip several months back where I dug 7 silver and 8 or 9 Indian head cents but only found 2 wheaties. BUT, the finds spoke for themselves so I stayed
Also remember, In the "hunted out" parks, most stuff still left will not give you a "clean" signal. Target masking will definitely be present, so slow down and investigate all the "silver" sounds. If I get a decent signal one way and at least a blip or broken sound when checking at a 90 degree angle, I dig it. Only when I get a complete null on the 90 will I probably pass it up.
I hope this helps a little and gives you a guideline to maybe figure out a new park you are trying