One of the most basic ways to date something like a spearhead is through stratigraphy. Stratigraphy works from the understanding that if two things are buried in the same place at the same distance down, they are likely from the same time period. I am going to assume that you are American, so I will caveat my explanation by saying that I am not in any way an expert on how the dating process works with ancient American artifacts. In my area of study, Ancient Egypt, we have evidence of robust trade throughout the region, and pieces of pottery are often found in any site we excavate. Because shape and decoration of pottery changes based on the general fashion of the time (for a more modern example, think about the water bottles people carried five years ago, in comparison to the ones typically used today with the whisk inside), finding a certain style of pottery in a location gives archaeologists a lot of information about the time period of the rest of the items at the same stratigraphic layer.

Stratigraphy is also used in geology, though this is also not my area of expertise, so I cannot tell you precisely how much overlap there is in useful information between the two.

As to how it is possible that a spearhead survived that long, spearheads are typically made out of stone. Chert, which is a type of flint, is a typical mineral used in stone tools. It is relatively easy to shape, using another rock to chip pieces off, but it is still a rock. It is no more or less likely to survive than any other rock, which is to say that unless it is in an area that is likely to be eroded away, there is no reason to believe that it would be destroyed over time.

I hope someone who is more of an expert on dating of stratigraphic layers in North America will come by to refine the answer for you, but knowing how things go here, I felt it best to at least offer an overview of how stratigraphy works in case no one does.

Hi, while there are some archaeologists specializing in that era here, if you don't get answers, try x-posting to /r/AskScience, /r/AskAnthropology, or /r/Archaeology (not a Q&A forum, but full of archaeologists)

Wow I had no idea about any of that, great insight. Thank you!