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.