There are monitors that can be worn for seven days, or longer, that give you a better chance at capturing the event. Sometimes even these monitors miss episodes that occur only a few times a month. If the event lasts long enough, there is a portable event monitor that you carry around with you and place on the chest as you are having the palpitations. This works the best if the episodes last for at least 20-30 seconds, because it take a few moments to get it out and activate recording.
Finally, there are small monitors that can be inserted under the skin overlying the left chest that can monitor the heart for up to three years. This is a simple procedure but still invasive, and is usually reserved until other methods have been exhausted.
With these tools available today, in most cases it is a matter of time before the cause of palpitations is eventually figured out.