The location of and old out house is fairly easy to find.
Look for a depression in the ground, that has a lot of nice vegetation growing up and around it.
Usually off to the side or the rear of the foundation of the main house. Look for what might have been a path leading away from the main house that leads towards or to a depression as mentioned above.
The tools I use are a shovel, a small gardening trowel/spade, a gardening claw and some kind of classify device/screen. And gloves of course.
Just be careful when you dig, dig slow and easy most of the stuff you will find is fragile (bottles etc)
Dangers? A cut finger from broken glass if your not wearing gloves. And an occasional biting and or stinging insect that like's the rich soil.
Finds? Coins, bottles, knives, bottle openers, keys. Expect to find anything that one might expect someone to carry in their pockets or on thier belt (with the exception of the quart whiskey bottles of course).
But mainly bottles. I assume from the large number I have found over the years that the out house was the place to hide ones drinking habits from the lady of the house or just to keep you warm on a cold night.