How can we know that we are asking Allah for something of which we have no knowledge, and what are examples of that, and how can we avoid it?

The Almighty's saying: ﴿So ask Me not concerning that of which you have no knowledge﴾ indicates that it is not permissible for a person to supplicate to Allah for something without knowing the permissibility of supplicating for it. The verse indicates that knowledge of permissibility is a condition for the permissibility of the supplication. This includes prohibited supplications, those that contradict Allah's definitive laws, and asking for things Allah does not do, such as asking for immortality until the Day of Judgment, or the removal of human necessities, or knowledge of the unseen. If the matter is ambiguous and one does not know the permissibility of supplicating for it, then one must refrain from it until its permissibility is verified by asking the people of knowledge.