If you dont want to believe you have it then nothing will convince you otherwise. If an official diagnosis will help, then a psychiatrist or psychologist. Otherwise, it's a matter of figuring it out yourself when you're ready.

As Kitten stated, a psychiatrist or psychologist can diagnose mental disorders. They do so by identifying any symptoms of said disorder. You've stated you don't believe & don't want to believe you have PTSD. I would encourage you to think about why you don't want to believe it. Is it stigma, fear of treatment, etc. There are people, like me, who have PTSD & have for some time now... & we can answer any questions you have. PTSD is the result of trauma. Some traumas, we can easily move past & do not develop PTSD. Other traumas may cause long-term effects. Being diagnosed allows you to receive treatment, whether that be therapy &/or medication. If you are concerned about/disturbed by your emotions, memories, intrusive thoughts, reaction to memories of the trauma, then seeking a diagnosis can help you manage & cope with those concerns. Best of luck