Always ask first.
This is taken from the Mass , gov site and is in the CMR's or code of Massachusetts regulations.
304 CMR: DIVISION OF FORESTS AND PARKS
12.11: continued
(3) No person shall use or offer for use metal detectors on Department property except at the
discretion of the Area Supervisor on designated swimming beaches and designated campsites.
I work for the Department of Conservation and Recreation (DCR) and I know of some parks that may discourage MDing due to the fact the hobby is misunderstood. I think the park employees have this pick and shovel mentality that we'll come in and tear up the place. Most employees , supervisors and park rangers have no clue as to the CMR's regarding metal detectors.
If you are planing to hunt state parks or beaches print out a copy of the CMR and ask for the supervisor on duty. Show him or her the CMR and say that according to this I need your permission to come in and remove the pop tops, pull tabs, screw caps and other trash I find with my metal detector while leaving no evidence that I was ever here.
If you come to my park I'll tell you to go have fun and might even tell you the oldest part of the park that might produce some silver. On second thought ...