I have a Carrier gas/ ac system installed in 1998. I just bought honeywell digital thermastat that requires a "common wire". I have blue, tan, black and orange not being used. How can I test to determine which is "common" ?

Typically you will have a red and a blue wire coming off the secondary of the transformer at the furnace. The blue may run to a terminal marked C and then to one side of the heat, cool, and fan relay. The red will run to the thermostat to a terminal marked RC. The thermostat then switches the red to white, yellow, and green as called for. The blue wire should be the common. The common will have about 24 volts AC with the red one. The 2 will have a low resistance between them with everything shut off.

If the old thermostat was a simple mechanical device, it didn't need the common. The digital will need it to complete its power circuit.