Omens need to be felt and analyzed. Each omen is specific to a person. Many omens occur more than once before the actual event occurs letting the person know of the impending event. If we consider Duryodhan's omen described in the first post, https://en.wikipedia.org/wiki/Gandhari_(Mahabharata)#:~:text=After%20the%20birth%20of%20her,regarding%20his%20firstborn's%20possibility%20of

Firstly, Duryodhana's cries, followed by winds and fires and the howling of animals. Each of these events taken individually would not mean anything but stringing them together caused people to recognize the omen.

The way to recognize a omen is to become aware of a natural event which has a lower probability of happening. For example, when driving from one place to another, you might catch a couple of red lights. But if you catch a dozen red lights or more consecutively, then it is more probably an omen. Once you recognize this omen, you should be able to confirm it with other low probabilty events before the actual event happens.