These shirts are mostly from the 1980s when Michael Jordan put Nike on the map. Of course the easiest way to recognize one these shirts is the color of the actual inside tag being blue. The most basic design is the word Nike written across the chest with the swoosh underneath.
Here are a few other examples:
So next time you are checking through the T-Shirt section keep an eye out for these. You can also search for these on eBay by checking vintage t-shirt lots. In your title search along with the keywords "vintage t-shirts" include the word "Nike" and check off the include description box to the right. You never know but someone may have put a blue tag shirt in their lot not know it's true value.
Here is my best thrift store t-shirt find to date. Not too shabby...
Check out some of my other posts on thrift store t-shirts.