How much does it take to diagnose an arrhythmia?

Capturing it: As a clinician you can strongly suspect it from a patient's description of symptoms but to accurately diagnose it you need to be able to record a symptomatic episode with some form of ECG (there are many modalities available that allow recording episodes even remotely).

EKG: Like any diagnosis, the history and physical provides clues, then, ideally, the arrhythmia would be present on a 12-lead EKG; however, the way an arrhythmia starts & stops can be recorded on a heart rhythm monitor. Some monitors are worn for days to months. Implantable monitors look for years. An invasive electrophysiologic (EP) study can provide critical details. See heart rhythm specialist.

Arrhythmia: there are several methods from ECG to Holter and even sometimes implantable devices to diagnose an arrhythmia ...