RAI KILLS EVERYTHING - cancer that is.
Anyway, your surgeon is right, the RAI questions go to the endo. They just take out the thyroid and take a few nodes for samples. Don't get too wrapped up on the node thing. If it has spread the RAI will take care of it. See if you can find a suport group where you live so you can talk to other people.
The answer to your questions - how do they know if it is cancer is simple. Once they remove the thyroid and some nodes, the pathologist will dissect it and determine if it is cancerous. Even if they leave some nodes in that might potentially be cancerous, the RAI will kill it. It is not advisable or prudent to take every node out of your neck or surrounding area. Too many important nerves and veins. The dosing of the RAI will depend on what the pathologist finds. Most everyone with pap cancer received the 150millicuries of RAI so it doesn't come back. But everyone is different.
So, the surgery is in two weeks. How are you holding up. Try to relax and prepare for the recovery.