A thermocouple can be identified by the color of its wire insulation. For example, in the United States a type J thermocouple grade wire has one red wire and one white wire, typically with a brown over jacket. A type J extension grade wire also has one red wire and one white wire, but it has a black over jacket.
As a general rule the red wire of a thermocouple or extension wire is negative and the positive wire is color coded according to the type of thermocouple. Different countries use different color codes.