This is a difficult question best suited for a forensic scientist, or an archaeologist. Firstly, the question to be asked is how old is the bold. Secondly, how old is the sidewalk? If these can be determined and are the same, then a good guess is that the newly bought bolt was hammered into the sidewalk.

How would the ages of the bolt and the sidewalk be determined? If it is a concrete sidewalk, a cement chemist would be able to tell the age from the way the concrete looks under an electron microscope.

The age of the bolt, one might be able to tell using a suitable dating method.

On the whole, it is a very difficult problem to answer in the general case. In the specific case that the bolt looks somewhat new, one could go to Home Depot and look for similar bolts, and then go on from there.