Long use can sometimes ripen into a legal right of access, and the doctrines that do it vary a lot by state in both name and requirements. It's not something you can confirm from a phone call or a parcel viewer, and a buyer's lender or title company generally won't accept it either. Whether it applies to this parcel is a question for a real estate attorney licensed in that state.
What you can check yourself: pull the deeds in the chain of title at the county recorder and look for a recorded easement, which is a written right to cross someone else's land, usually described by metes and bounds and referenced in the deed. If it's recorded, access is a documented fact. If it isn't, you're buying a dispute with the hayfield owner and hoping.
Access is the single most common reason cheap rural parcels are cheap. Landlocked acreage often sells for a fraction of comparable land with frontage, and the discount is real rather than a mistake in the market.
The piece that catches new buyers is the resale side. Even if you're comfortable holding it, your end buyer's title company will flag the missing access, and buyers who want to camp or build will walk. Before you offer, find out what the neighbor would want for a recorded easement. Sometimes that conversation is a few thousand dollars and turns a $14,500 problem into a sellable parcel. Sometimes the neighbor has wanted your 20 acres for years and says no on purpose.
