Worth repeating: Laws vary widely from state to state, and court rulings may provide legal precedents not readily apparent in a reading of the statutes.

Here in Colorado trespassing is pretty much a "strict liability" offense. It is the responsibility of each person to avoid entering into or remaining upon any private property without permission of the owner or person having lawful control. Fences and signs are not required for enforcement. Trespassing offenses vary considerably depending on the nature of the property (agricultural, residential, commercial, enclosed, dwelling structure, motor vehicle, etc); example: trespass on unenclosed residential property is a low-grade petty offense, on agricultural land is a misdemeanor, entering a motor vehicle may be a felony.

As a general rule the use of physical force in defense of property is prohibited, and use or display of a firearm in defense of property is likely to subject the property owner to greater penalties than the trespasser will receive.

Signs, fences, and gates are still a good idea, providing at least a presumption of intent to trespass by any who venture through such barriers.

My home is within a private HOA community. All of the streets, sidewalks, and common areas are private property. The property is completely enclosed by fencing. A sign at the ungated entrance advises "PRIVATE PROPERTY - OWNERS AND INVITED GUESTS ONLY". Anyone else who steps from the public roadway into our property is committing Third Degree Criminal Trespass (a petty offense subject to monetary fines but no jail time). We still see joggers, dog walkers, cars driving through, and an occasional salesman or solicitor. I have asked several to leave and only one has given me any argument. Cell phone cameras seem to make an impression on a lot of folks.

One annoying couple from a nearby residence liked to walk their dog and leave piles of droppings, got pretty mouthy when I objected, so I watched while they walked home, pulled county real estate records to identify them, sent a certified letter warning against any future entry to our property along with a copy of the state trespassing statute, and a photo of their dog doing his business while they watched. No further problems.

Caution is the best policy. We never know how some people will react to anything said or done. Better safe than sorry.