Proving a negative can feel impossible. You know you did not buy something, but now you are being asked to prove what you didn't do.
That is one reason the surrounding facts matter. Where were you? Where did the transaction happen? What was purchased? Was anything delivered somewhere else? Does the activity resemble the way you normally use your card?
There is rarely one magical piece of evidence. Often the story comes from putting the facts together.
If your credit card company denied charges you know you did not make, read “These Credit Card Charges Aren’t Mine. Why Was I Denied?”:
https://www.cardozalawcorp.com/blog/these-credit-card-charges-aren-t-mine-why-was-i-denied-.cfm
