To differentiate cardiac pain from the left arm at younger age, one should see if the pain in arm aggravates on arm movement. If the arm movement causes discomfort, then it's non cardiac. If the pain increases on running or climbing stairs then cardiac cause must be evaluated. Would always suggest to get ECG and Echo done atleast once to be sure.

You need explain duration, type, location of pain or any association with exercise . Better to show to cardiologist and do simple ecg first