How to locate the owner of a 5' styrofoam wing found in a field?
If it has a US registration number on it (N followed by 1-5 numbers or letters) you can search the FAA registration database to find out what aircraft has that particular registration number, and who owns it.
In the case of a 4-5 foot styrofoam wing this would likely only be useful if the aircraft in question was a registered UAV.

Some common sense needs to prevail here: Model aircraft often have an N number painted on them for decorative reasons, so if you look up N12345 and it says "That belongs to a Piper Cub" you can safely assume this wing does not belong to that particular real-world aircraft (as the Cub's wings are not made from styrofoam, and are more than 5 feet long).