All kinds — literally — of apples are used as apple rootstocks from seedlings through virus-free Cornell clonal offerings, so you never know what suckers are springing up under your neighbors’ trees. People will guess, however. You just need to buttonhole a “local expert” who can integrate a little knowledge about likelihood of where the trees were ordered and when they were planted with what the rootstock should look like and the apparent overall size of the parent tree.
Although there’s a great variety of dwarfing rootstock out there nowadays, the types typically used commercially twenty years ago are probably fairly limited so any guesses are likely to be good ones. For instance, red suckers are most likely to be B9. See the exchange of views pro and con about this assertion at “Bud 9 Bloom.”