There is another test that can be added to TomB's list: Ask an AGW proponent to list 3 good things resulting from a warming earth.

Virtually every imaginable change has trade-offs; any list of disadvantages is offset by a list of advantages. It's the balance between these sides that determines if a change is beneficial or harmful. AGW is the only theory I can think of that is devoid of a benefits list. This is unheard of in scientific debate but quite common for ideas in the political arena.

If you ask an AGW proponent and he won't acknowledge a warmer earth has some benefits, you know you will be engaging in a political debate. Scientific arguments will be ignored or ridiculed even while the proponent insists it's about science. The result of the test indicates to treat what ensues as a political debate, there will be no give and take fairness you expect from a scientific discussion.