GetDefinitionId.cs

public static void GetEventDefinition(Item item)
{
    Assert.ArgumentNotNull(item, "item");

    var field = item.Fields["__Tracking"];

    if (field != null)
    {
        var trackingField = new TrackingField(field);

        foreach (var events in trackingField.Events)
        {
            var definitionId = events.DefinitionItem.ID;

            // Perform any logic here
        }
    }
}