basically just stay away from any cogs that are stamped from a single piece of flat metal, any whose threads are visibly uneven or don't look well machined, or any whose threaded flange doesn't look as wide as the others. dura-ace is a great cog for the money, but has limited sizing. EAI and phil are fine cogs but a little pricier because they are nicely finished.
personally, I didn't trust the surly cogs when i saw them at the LBS. I've run dura-ace and the Blue Collar (milwaukee) on mine and they're roughly comparable in quality and price.