If they were treated, they could still have a lot of chemical in them and could be a source of leaching and contamination. The question is: Were they treated in the first place?

If they were commercially treated, it's possible that there might be incision marks on them (which allow for deeper chemical penetration). This said, the absence of these marks does not mean they weren't treated commercially or that they weren't treated by a do-it-yourselfer. You could cut through one of them to see if there is any evidence of a different color to the wood around the outer perimeter. Unfortunately, however, no visual inspection is going to give you an absolute answer. The other thing you might look to is what condition the posts are in that went into the ground many years ago (if any remain). Lack of decay below-ground might suggest that they were treated.

Those are all of the indications I can suggest. If you want absolute certainty, the best thing would be to take a sample to an analytical chemical: a decent lab should be able to test for the presence of arsenic for not a huge amount of money.