When asserting a claim for trademark infringement, it is necessary to show that you own a valid mark. In the previous post, Johnny Manziel would not be able to show ownership of a valid mark without sacrificing his college football eligibility at Texas A&M.
Registration of a valid mark is prima facie evidence of its validity, which means that a lot of the heavy lifting is already done for the plaintiff. It would be up to the defendant to argue that the mark is invalid.
Following demonstration that a mark is valid, the plaintiff would need to show that there is a likelihood of confusion between the plaintiff and defendant's marks. The various courts of appeal throughout the United States basically use a version of the same test. First, the court looks at the strength of the plaintiff's mark. The stronger the mark, the easier it is to show infringement. If the mark is barely registrable with the United States Patent and Trademark Office, the plaintiff has more work to do to show infringement.
Second, the court looks at the the similarity of the plaintiff and the defendant's marks. Obviously, the more similar they are, the more likely there is infringement. Third, the court looks at the competitive proximity of the plaintiff and the defendant's products. The closer they are to the same industry, the more likely there is infringement. Fourth, the court looks at whether the defendant intended to confuse the public. Fifth, the court examines whether there is actual confusion among consumers as to the source of the products. Sixth, the court looks at whether customers of the markholder exhibit care in selecting that particular item. If customers do not take much time in deciding whether to purchase a product, the easier it is to confuse customers.
If you purchase a car, you are likely to notice that a defendant's mark is confusingly similar, because you take care to notice exactly what you are purchasing. But if you purchase a box of crackers, you do not take nearly that much time or effort in examining the purchase. You may just grab a box based on its look, and later find out that it is not the brand you intended to select. Infringement depends on how the court weighs these factors.