Non-pecuniary externalities can be described without any reference to prices. A corporation moving to a small town might cause traffic to worsen, increasing commute times. This is a non-pecuniary negative externality and may lead to a deadweight loss. That same corporation might cause rental rates to increase. This is a pecuniary externality and doesn't lead to a deadweight loss.
Identify a negative (non-pecuniary) externality and describe the nature of the cost imposed on third parties.
Identify a positive (non-pecuniary) externality and describe the nature of the benefit received by third parties.