Tinky, I can give you some quick tips. I'll try and keep it simple. If there's any additional interest on the site, perhaps I'll write a step by step article, with Carol's permission of course.
Given the choice, you want to look for privies whenever possible. Since most folks didnt have trash service or couldn't afford it, table scraps, bottles, broken china were typically disposed of in the outhouse. Fortunately for us, the um.....liquids in the privy cushioned anything breakable.
Trash pits, on average, aren't as productive. Smaller holes were dug allowing less space. Trash was often burned, turning anything inside the pit in to a molten mess. Finally, bottles were often intentionally broken so that all of the dirt dug out of the hole could be returned without creating a mound.
Older city lots with clearly marked boundries are easiest to begin your search. Farms can be more difficult as site selection for the outhouse wasn't constrained by property lines, alleys, etc.
When arriving on any site with a building, house or foundation markers, you need to set down all of your gear and take a hard look. Put yourself in the original occupants place. Where was the common front entrance to the house ? Where was the back door ? Was there a carriage house or an out building ? Where did the occupants cook their meals? Depending on the area of the country in which you live, many pre 1890 homes had external kitchens. Where did the o.o. s (original occs) leave the property from the rear ? Any deductions made about understanding how people used the property, will be potentially beneficial to both MD ing and digging.
Once you think you have a general understanding of the manner in which the property was used, it's time to break out the common sense.
Again, put yourself in their shoes. Common sense dictates that the privy is going to be behind the property. Who wants to wave to the neighbors as they dash out in to the FRONT yard to head to the reading room ?
Now, think about the aroma of a porta potty. You sure as heck don't want that vile smelling thing near the back door. Even with people dumping bags of lime into privies, they were odiferous at best.
Think about the weather ? if it's 2 am and you need to make a run for the border, you dont want it so far away that you have to trudge through much snow. Even if the family is using chamber pots, someone still has to carry their toxic cargo to dispose of it.
So, if it were you, you want it away from the house , but not so far away that you have to saddle up to get there ! Most outhouses/privies are typically 30-50 feet from the rear entrance of the home. This same logic can be used for farms or large unmarked plots of land. I used to take a commercial tape (200 feet) lay out 50 feet and mark a semi circle from the rear entrance of the building/dwelling.
Now stand where you think the rear of the dwelling was, and look some more.
If you have a rear entrance to the property, you wouldn't want your outhouse near it. Who wants to huff fumes every time they come home?
So, now we are looking for a spot behind the house 30-50 feet away from rear entrance and not near the rear property entrance.
Here's where you need a special tool. Purchase a quality spring steel digging probe. 36 inches is the the standard. Most serious bottle collectors have 36, 48 and either a 60 or a 72 inch probe. A 36 will meet 85% of your needs.
A probe is basically a 1/4 inch in diameter piece of spring steel with a T handle on one end and a triangular tip on the other. Build quality must be high as you put a tremendous amount of pressure on the handles. I know of two people that have been speared through and through by poorly made probes when the weld at the handle failed. It was not pretty.
The probe is used by pressing it in to the ground. You are feeling the ground consistency. Once you have a feel for ground which has not been turned, you will recognize any area which has previously been disturbed.
Depending on lot size, and assuming you are working in town, start in the rear corner of the lot AWAY from the rear entrance to the property. Locate the rear property marker or general guess and then probe the area 2 feet from back line and two feet from side line.
What do you feel as you put pressure on the probe ? Does it take consistent pressure all the way to the knuckles ? Do you feel inconsistency, or hitting small bits of trash ?
Remember that probe tip I told you about ? It plays a number of roles if used properly. Just looking at it, you would assume that the slightly bulbous end was used solely to make a slightly larger hole, lessoning the friction on the rod. While this is true, there is another huge opportunity to learn something.
A properly designed probe will capture a dirt plug from the bottem of your probe depth. That dirt plug can tell you a lot about whats in the ground.
So, start probing. Go slow. You want to apply just enough pressure to drive the probe in the ground. Smooth consistent pressure. If you hit an obticle, STOP APPLYING PRESSURE AND IMMEDIATELY REMOVE THE PROBE. Too many people starting out force the probe, and typically drive it right through a bottle.
Now look at the plug of dirt. What do you see ? Is it just clean dirt ? Is it the exact shade of the yard ? Or is it a different color ?
Orange, rust colored, or small rust colored flecks usually indicates that iron is likely buried. A great sign that requires additional investigation.
White or whitish flecks are another great sign. White could be an indication of of ash or more importantly, a sign that lime has been dumped in the hole. As stated above, lime was used in outhouses to help knock down the smell. Again, further investigation is required.
Black, or blackish flecks. Again, very promising. Black could be an indication of a fire pit, or the remnants of stove embers.
So, we have hit something underground but not sure of what it is. Move your probe over three inches and probe again. Now, really think about what your hands are feeling. Do you feel or hear rice krispies ? This is typically caused by small bits of broken glass. Anything in the top six inches is usually nothing more than surface crap which has worked it's way in to the ground. If you are getting rice krispies at 18 inches or deeper, you have usually found a trash pit or privy.
Now you need to dig a test hole. 12 inches by 12 inches, by 18-24 inches. Dig slowly and pay very close attention to every scoop of dirt that comes out. No need to pay much attention to first six inches or so as that is typically surface related.
You want to dig a hole with smooth clean walls. No hacking or shopping. In addition to watching the dirt removed from the hole, you're examining the hole walls for anything out of the ordinary. Do you see water seep lines ? An agled deliniation of ash or soot ? Iron, metal or rust bleed lines ? All these are indications that the ground has been disturbed.
This should get you started. I have left a ton of stuff out which can save you a lot fo wasted time. In addition, I failed to address the whole research aspect of a site which can save you hours or days of wated effort. Sorry for running off at the keyboard.
If you have a specific site in mind, PM me. I can do some work on the net and likely save you some grief. Good luck !