How do I know if my old thermostat contains mercury?

The thermostat will have a glass tube with a silver liquid inside of the tube. Mercury is typically found in older mechanical thermostats like the Honeywell Round T87 and rectangular T8090.
NOTE: Do not place your old thermostat in the trash if it contains mercury in a sealed tube. Contact the Thermostat Recycling Corporation at http://www.thermostat-recycle.org/ or 1-800-238-8192 for information on how and where to properly and safely dispose of your old thermostat.