What is the first step I should take to determine if my mom was a victim of medical negligence?

I am sorry to hear about your mother's cancer diagnosis.   First, I would request her medical records. Start with at least a year from the cancer diagnosis but get more depending on when you believe the failure to diagnose/delay occurred.  It will take some time, likely 30 or more days to get the medical records.  While you wait on those, I would start calling local attorneys in your area who practice medical malpractice/negligence cases.   You should be able to get a consultation for free without much difficulty.  Please note that your mother's potential claim is likely limited by a statute of limitations so it is important that you act promptly.  Best of luck. NOTE:  This response is general in nature and should not be considered legal advice.  No attorney-client relationship exists or is formed by this response.