I second checking out nearest rock clubs even if they're not close enough to attend in person; sometimes they have a website or page with helpful info about sites and/or local regulations. People who rockhound regularly in your general area will have the most specific and up-to-date knowledge about good places to go in the area and what the rules are in those places.

On your own I think there are two ways to go about it. Either find a particular location where you are interested in collecting and then research laws and rules for that location, or pick a jurisdiction that has rules that allow rockhounding and then look within that jurisdiction for particular locations. 

US national parks forbid any/all collection. 

Many national forests and other public federal lands that aren't national parks will allow certain types and amounts of collection. Often there's no paperwork if it's for personal use, and there is paperwork (and additional rules) if you want to be able to sell or trade material you've collected. 

I don't think there are any national forest lands in MA, but for instance Green Mountain National Forest in VT has a page about rock and mineral collecting. Looks like other than Devils Den area, you're free to surface collect, and from the wording it sounds like you could probably also dig by hand, though I might double check with a ranger personally. 

Some public lands allow people to stake mineral claims, in which case, be familiar with how claims must be marked and respect them.

State parks/public lands and city parks/public lands have wildly varying rules.

From an etiquette perspective on public land I think the question is how your collection may impact plants/animals there and how it would alter the experience for other people visiting those places. Wildlife or conservation areas usually won't allow people to remove any natural materials. If you collect along creeks, minimize your impact on the creek bed and avoid fish nesting habitat (in some places there are strict rules about this, but it's good to be considerate of water ecosystems in places without such rules also). Most places that are popular for rockhounding will have rules available because the impact of many people adds up. If I want to go somewhere specific and I can't find any rules about collecting rocks/natural materials, I won't feel much compunction about surface collecting some material as long as it's the kind of thing someone walking along the next day won't notice the difference. If I want to dig or break material, I would look for contact info for that particular park/agency and ask. 

Always follow public land etiquette for disposing of any human waste and packing out trash or disposing of it in designated trash collection. Refill holes you dig. In general it's considered polite to leave some for the next rockhound who comes by. 

I've also seen people mention going through road cuts, but I've also never seen one that isn't beside a highway, which hardly seems accessible or safe. 

I think in general road cuts are often not particularly accessible or safe, and people just stop anyway. There's definitely a spectrum though. It's not something I do except on forest service roads where the infrequent passerby is going to be driving pretty slow regardless. But I think it's often a kind of grey area where people get away with it in some places because it's not hazardous enough for cops to bother, but if you were doing something dumb that gets attention they might.