Does an ecg show abnormalities even if you are not having palpitations at the time?

It can: Palpitations can be caused by a number of heart rhythm disorders. Sometimes these abnormal rhythms are present even when you don't feel palpitations. Therefore, the first and easiest test is an ekg.

Yes, it can: Some patients are not symptomatic, and yet can have abnormalities in heart rhythm, such as atrial fibrillation or pvcs. Often times, an event monitor or holter monitor can be helpful in identifying episodes over a longer period of time and correlating with symptoms.