OK, let me set the record straight about old outhouses. I have dug outhouses for over 35 years, and they are a fantastic time-capsule! I have assisted the local archaeologists for years, and an old outhouse is one of the best places to dig. If you are wondering what can be in an outhouse, well, let me tell you. ANYTHING that can fit in the hole can, and usually has been found in them before. You name it, its been found. Sometimes, something would accidently "fall" in, while other times, they were used to get rid of trash. Now, keep this in mind, what was considered "trash" years ago, could be a real treasure to the digger, or property owner. Way back, when outhouses were common, the better ones were brick or wood lined, with a brick bottom. When the hole became full, they would call a company to scoop out the contents, making the outhouse like new again. This is where the term "honey-dipper" actually came from. The "probe" that was mentioned is commonly known as a bottle probe or probe rod. The shaft was usually made of spring steel, with a "T" handle welded to the top. These are used to probe the ground. When an item is hit, you decide if you want to dig it. When you get good using one of these, you can tell what the "struck" item is made of a lot of the times.
Most houses built before 1900 had an outhouse. Sometimes in a neighborhood, there would be common usage outhouses located in the middle of the block. Sometimes, there would be more than just one located there. These are commonl;y referred to as a "2-holer", "4- holer", etc. The best method of digging one is to sift all the dirt inside the hole. If any one is interested on how to build a sifter, I'll be glad to post some photos of mine. They're not difficult to make, and you might be surprised with what you find. A lot of the time, an outhouse location can be found by looking for a dip in the ground. Using a probe rod (bottle probe) you can determine if anything is down there. If a homeowner finds their old outhouse, and they are interested in the history of their place, digging it is a MUST! Where I live (tampa), there used to be an old military fort dating back to the 1820's. I got into the officers quarters outhouse, and found an entire bowl and pitcher set from the 1820's. Most of it was broken into pieces, but after glueing it back together, it makes a great display, that any museum would love to have.
OK, now here's a word of caution. Some of these outhouses can be quite deep. BE CAREFUL!!! The possibility of the side caving in are there. Even though this is quite rare, it has happened. Of course, be careful with the glass, it will still cut you easily. If you are sifting the outhouse, you wouldn't want to throw the bottles and large items in the sifter, these you can pull out one at a time. But, by sifting all the dirt, you'll find EVERYTHING that was ever dropped, or tossed down the hole. Also, as a rule, there will be no remains left of the um...well....the um....crap. Sorry, didn't know of a better way to say it.
Always wear gloves when you are digging, and be sure and keep EVERYTHING you find that you are not sure what it is. You can always throw it away later, but if would be impossible to go find it again, if you find it was of value. At my house, I have a museum set up in a room at my house, that contains everything of interest I have found on my property.
Here is just a small idea as to what can be found in the outhouse. Bottles, china (dishes, plates figurines etc.), doll parts, coins (copper, silver and sometimes gold), tokens, clay pipes and pipe stem, buttons, musket balls (the old bullets), clay marbles, jewelry (costume as well as gold & silver), and the list just keeps going on and on. Not every outhouse will have these items, but the chances of some of these items being found is good. Keep this in mind, when an outhouse was "taken" out of commision or service, it was a common pratice to fill the outhouse with trash to help fill it up. So, by looking at what comes out of what layer may give an indication as to when the outhouse was used, as well as filled in.