It is possible for a seasonal pier to be "grandfathered". Seasonal docking structures that were installed prior to September 4, 1978 are considered to be grandfathered provided they have been maintained in the same size, location, and shape since that date are were not removed for a period of 5 years or more. If you don't believe the structures meets this criteria then looking for a permit with the registry of deeds as codeman suggested. Unfortunately not everyone is diligent about recording their permit when they get it. If you don't find anything at the registry you can also check Wetlands Bureau records directly by going to the wetlands website at
www.des.state.nh.us/wetlands and clicking on the "permit query" button. Good luck