How can I detect cold starts in AWS Lambda?

Is there a clear way to identify "cold starts", either in runtime in the Lambda itself, or via the logs? I know that cold starts are characterized by longer runtimes, which I can actually see, but I'm looking for a clear cut way.

I'm using Node.js if that matters.

Update: There are two good answers below, for two use cases:

• Identifying the cold start as the lambda runs.
• Identifying the cold start from the CloudWatch log.

If you add some initialization code to the top of your NodeJS script, you will be able to tell in the code that it is a cold start, and you will then be able to log that if you want to see it in the logs. For example:

var coldStart = true;
console.log("This line of code exists outside the handler, and only executes on a cold start");


exports.myHandler = function(event, context, callback) {
  if (coldStart) {
    console.log("First time the handler was called since this function was deployed in this container");
  }
  coldStart = false;

   ...
   
  callback(...);
}

Update:

If you only care about seeing cold starts in the logs, Lambda now logs an extra "Init Duration" value in CloudWatch Logs for cold starts.