Should I be worried if I fainted?

Before you actually faint, you might experience warning signs like lightheadedness, nausea, buzzing or ringing in the ears, tunnel vision, or feeling sweaty and clammy. — dpa

You may have heard that people who feel faint should sit with their heads between their knees, but is that true?

And how can you tell whether you or someone else who has fainted should go to the emergency department, or perhaps even see a cardiologist?