Compostable plastics will be labeled as compostable. Unlined and uncoated paper-based products are compostable, with the paperboard clamshell being the most popular.
Shiny paperboard typically is a sign that it is plastic-lined and NOT compostable.
Look for the certification label from the Biodegradable Products Institute (BPI - see image below). BPI World certifies whether products are able to be composted in commercially run composting facilities.
You can also Tweet or email a picture of the item in question @CvillePW or firstname.lastname@example.org .