Personally, I would buy an Argetine from Varnyard. He has great tegus selectively bred for temperament and you won't be getting ripped off.

Otherwise, the Argentine hatchling usually has a greenish head and two loreal scales between the eye and the nose (as a rule). The Colombian feels smoother and has one loreal scale (as a rule). The Argentine also has a thick horizontal stripe along the shoulder.

Google some images to get familiar. There are a lot of stories of pet stores selling the wrong type of tegu. I'm sure some are rip-off scams and some are just cases of mistaken identity.