What is a miracle?
A miracle is an event which is beyond the ability of any human person to perform.
How can we know if any given event is a miracle or not?
This question cannot be answered until a more basic question is answered.
What is this more basic question?
The foundational question here is whether miracles are possible in the first place.
How can we arrive at an answer to that question?
Our answer to this question depends on our worldview. Those who hold to an atheist worldview must answer this question in the negative; miracles are not possible in an atheist worldview. On the other hand, miracles are possible in a theist worldview since it is always possible for God to do something that is not possible for a human person to do. These acts of God would be miracles.
In a theist worldview then, how would we know which event is a miracle or not?
We would examine what happened and try to determine if it is beyond human ability or not.
What is the greatest miracle which ever occurred?
The resurrection of Jesus is the most important miracle claim in history. The truth of the Christian religion stands or falls with the truth of the resurrection miracle.
How does Locke define a miracle?
“A miracle then I take to be a sensible operation which being above the comprehension of the spectator and in his opinion contrary to the established order of nature is taken by him to be Divine.” source