Sometimes in the United States private land won't be marked clearly or there won't fences along the lines or maybe there aren't any "no trespassing" signs. Especially in the western parts with big open spaces this can be a problem.
Or sometimes the land can be public, but the rules can be different depending on the who owns the land and whether its say a Wilderness area or a National Forest.
What is the easiest way to tell who owns the land?
In order to avoid trespassing, you should know positively where you are. The easiest way to do this is with a high quality GPS with a public lands map package such as those offered by OnX Hunting Maps: https://www.onxmaps.com/. Relying on fencing, fence style, and other notices might work for well-traveled areas, but if you are off-trail, on a waterway, or other entry to private land that isn't usual, you may inadvertently enter private land. This can get especially confusing in areas where public lands are used for private grazing as often the grazing permit holders try to make public lands look private, which is bad for everyone.
In general, you will have a land ownership idea in mind, such as traveling a national forest, BLM parcel, or other public land. By knowing the boundaries and double-checking with your GPS, you can determine when you are passing out of land you know you have a right to be in. If you aren't sure where you are or where the boundary to the public land is, then stay where you are sure.
This is especially important when hunting or fishing, as sometimes heightened penalties apply to trespassers engaged in this activity.
I have a relatively large parcel of land next to a public piece of land and I am continually dealing with trespassers. Many are harmless, but some are not such as people accessing hidden pot growing operations, so my response is generally pretty gruff and often I am armed until I know which category people fall into, so expect to be treated as a potential threat until proven otherwise when you are found on someone else's land.
Keeping property completely posted is often difficult due to weather removing signs and people simply ripping them down. Fencing is also sometimes down for a few weeks before I notice. The onus is on the traveling party to know where they are and whether or not they have a right to be there in most of the Western United States. There are different requirements for civil and criminal trespass and generally criminal trespass laws require someone to remain after being asked to leave and/or proper signage. Civil trespass may not require such a heightened burden, but most people won't sue you for civil trespass if you haven't caused any damage.
In most states of the US, if there isn't a sign telling you to keep out, or a physical barrier like a fence, you are OK to go there, assuming you were legal where you started from.
Of course not all private land is posted with No Trespassing signs. That means you can end up on private land without even knowing it. Different states have different rules, but generally in that case you are not in trouble. The land owner can ask you to leave though, and you have to comply.
While this may sound tricky, it's really not much of a big deal in practice. Before you go hiking somewhere, you really should be familiar with where you intend to go and what land you will be on. The biggest problem I've run into out west is not accidentally ending up on private land, but having my way blocked by a private enclave that is not shown as such on the map.
I am very partial to GaiaGps on my phone. They offer a public land layer that gives you a color coded overlay of various land ownership types. It has been very helpful.
This has been a problem for quite a while, it used to be that one needed really good paper maps to be able to tell.
Now there are chips for GPS units that show the ownership of the land on the map (there are also apps for phones) with an overlay of the property types and boundaries. That way people can know where the property lines are and can avoid trespassing and are better able to access public lands.
These were mostly designed with hunters in mind (sometimes hunting areas are limited to public land or vice versa) but they could also be useful for hiking and backpacking in areas where there is private land mixed in with public.
There are a number of different makers of these chips (basically memory cards) / apps what you are going to want to search for to find them is
There are a now quite a few of these applications by a number of different companies. Its worth pointing out that the coverage can vary by area.
