Kerry,
Someone will correct me if I am wrong but here goes. My head & neck surgeon told me that they can palpate or feel, an enlarged node that is 1 cm (about 1/2") or larger and it is characterized by an "almond" shape. It feels different than a knot in a muscle (which we have lots in our necks, especially if you use a computor a lot.

MRI's and CT's can typically detect a tumor only down to 2mm (I don't know what their accuracy is in relation to nodes). Sometimes there are particular nodes that they will remove and test and they are known as "sentinal" nodes.

If they suspect a node they can also perform a fine needle biopsy.

I was stage III/IV and had no node involvement. I too was surprised but grateful, that there was no involvement.

Was his tumor well differentiated? Focally invasive? These are possible explanations why they didn't remove the nodes.

During radiation treatment they will often irradiate the nodes as well for prophylactic reasons.