Here are my suggestions:

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1. Look at aerial photographs. if your property is surrounded by people on small lots, they will be using your property for whatever they want and you are going to have a helluva time policing it. This is quadruple if you own the land right behind a development of homes
2. Look at the topography of land, and look for trails or dirt roads, are there a bunch of trails crisscrossing the whole area including your property? If so people aren't going to stop using them just because you own it now.
3. Look for neighbors you just KNOW are going to be a problem from the start. People with obvious obnoxious "anti-tresspassing" signs, like every single tree is posted on THEIR property line facing the road, but none anywhere else? probably an asshole who will trespass constantly and be a dick about it

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Finally, look at the general area you are looking to buy in, does it look like a place people you don't want are just going to randomly show up at because its right off a major roadway where it will be a short cut to another place or near a plaza or is it 15 miles from the nearest parking lot and the only way to get to it is a 2 lane country road?

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Trespassing signs don't work, gates and fences don't work, the only thing that will keep people off your land is having land in an area that isn't convenient for them to be able to get to