Head gasket

1999 5.7 Tahoe 4x4
I just hard my tahoe serviced for replacement of tansmisiion filters. The mechanic who did this told me that while under the vehicle he noticed rust on my freeze plug and some tacks, sorta white-ish that seem to be coming frome the head gasket. He indicated that I "may" need to replace the head gasket but didn't know for sure. He also said the freeze plug was rusted and needed replaced. I have No miss in the engine at all. The temp gage stays between at 150 all the time after it heats up. Thats okay. I am lossing a small amount of fluid for the past few weeks. I had the water pump replaced in 07. My question is this, how can it be proven out that it is indeed a head gasket before pulling the engine as he suggested? This is a very expensive job so I hear and want to know if it can be preven out first that it is what he says "may be"