You typically can't see your turn signal lights when you're inside your car. However, most vehicles have a subtle way of letting you know when one of your blinkers burns out: the indicator in your dashboard will blink faster than normal.
Picture: Wikimedia Commons
As automotive site 2CarPros explains, the indicator inside your car is designed to operate under a specific electrical resistance. When the bulb burns out, the resistance changes, which causes the turn indicator in your dashboard to blink faster. This is a quick way to know whether your light needs to be changed without getting out of your car.