There are a few good ways to get started. First, locate the oldest houses in your area. Anything built before 1900 is what I recommend, although there are nice finds in newer dumps too, like milks, toys, poisons, local druggists and sodas, beers, marbles, etc. You can use historical books like town histories, town directories, state registers, old newspapers, or early maps for instance. Try the library or book stores. Antique shops often have old books for sale as well. Also Google does have an archive search of old books and newspapers, which is pretty interesting. If your older part of town was built near a river, check up and down the banks. You can also ask older folks in town if they remember any dumping grounds. Generally, rivers were used a lot for trash disposal.
Most houses/farmsteads in rural areas took the trash they had and tossed it out back somewhere, usually into low areas, over river banks, or over a stone wall. Another good spot to look is under these buildings, or if you have access, in the attics. Crawlspaces under barns, ells, or other outbuildings, even in basements where root cellars were kept are very good spots. A lot of times there would be a trap door or loose floorboard which would be lifted up and trash was tossed. Out of sight...out of mind. Of course, the outhouse is a good place to dig. I usually can locate them in a corner of the barn or ell, or on the outside of the barn wall. Outhouses can be hard to find, so getting a 4' long probe will help. Probes are easy to use, and will sink very quickly into a filled in outhouse, because they are usually topped off with ash or other fill. The surrounding soil will have more resistance when you probe around. Some outhouses, however, had a removable box under them which was emptied periodically, so you won't find a filled in hole with those.
In an urban area, where homes were close together and yards were small, outhouses are easier to find and can produce great finds. They are usually located on the back or side property line. Sometimes they are right down the center, and are typically a short walk from the back door. Lilacs can be an indicator of their location. Most urban household trash was put into the outhouse, because they had to throw the trash somewhere, and the outhouse was an easy solution. In my area, the outhouses are usually 4-6" deep. In other parts of the country, they can be 20" deep because it was easy digging. Town dumps are great if you can find them, however they are not usually marked on maps. Early town reports and board of health reports can also be helpful in researching areas of town where "nuisances" (code for outhouse or trash smells) were present.
Most dumps you locate on the ground or on a riverbank will date from 1875- 1940's. Many times the dump will only date for a small time frame, like 1890's -1920. You never know until you dig it. Privys are usually where you will find earlier pontilled bottles, (1830's - 1870's) but there are always exceptions. Some folks used outhouses into the 20th century.
Check out the link I posted above in my first post. New Jersey has a fantastic and important history in early glass making, so finding info about Jersey glass shouldn't be hard. I'll post some links to other sites next.
