Since "No" means "absolutely none without exception" then it is hard for "No" statements to be true and easy for them to be false. Indeed it only takes one example to prove an E Form statement false. "No Americans admire Adolf Hitler" can be disproven by a single American. Find one who admires Adolf Hitler and the sentence is false. However, finding one, two or even hundreds of examples of Americans who do not admire Adolf Hitler will not be sufficient to prove the "No" statement true.
We call examples which disprove universal statements counterexamples. A counterexample to an E Form statement is a member of the subject category that is also a member of the predicate category. So to find a counterexample to "No dogs have blue tongues" we must find a member of the subject category a dog that is a member of the predicate category animals with blue tongues. So a Chow would be a counterexample. Notice that finding a member of the predicate category that is not a member of the subject category is not a counterexample. So if you find a lizard with a blue tongue that will not be a counterexample to "No dogs have blue tongues." The statement doesn't say anything about lizards. It only makes a claim about dogs.
While positive examples of E Form categorical statements do not provide proof, they do provide confirmation. Suppose I say "No South American nation has a Muslim majority population" and someone challenges my claim. I would naturally point out positive instances of my claim "Brazil is not a Muslim majority nation. Peru is not a Muslim majority nation. Venezuala is not a Muslim majority nation." While each falls short of proof, each example does provide relevant evidence that supports my claim. Such positive evidence that falls short of proof is called confirmation.
This is easy to see with such concrete categories as dogs and their tongues, but it is surprising how many mistakes people make with regard to counterexamples when the categories are abstract and hard to visualize in the imagination. But logic is about forms and patterns. It does not matter if we can visualize the objects talked about or not.