I bought a metal detector just for this purpose. It was not a cheap model, and when I was done, I got decent money for it at a yard sale. IIRC, the brand was White. There are a lot of prospectors around here, so it got a good price.
 
Once I located all four corners, I hired a fence company to put up a quality four strand barbless wire fence, 6" inside (my side of) the property lines on 3 sides, and cedar split rail along the front. The only "opening" in the property's fencing is our driveway. This kind of surprised some neighbors who were used to riding their horses or atvs across our property, when they found there was no exit at the other side. Put a stop to that practice, too.