This paper provides a novel method for detecting production bottlenecks and the shifting of the production bottlenecks. All production systems are constrained by one or more bottlenecks. Improving the bottleneck will improve the whole production system. Yet, finding the bottleneck is no trivial task. Furthermore, the system may change over time or due to random events, and subsequently the bottleneck may shift from one machine to another machine. The presented active duration method determines the bottleneck based on the duration a machine is active without interruption. The method is very robust, easy to apply and able to detect the primary and secondary bottlenecks in a wide range of production systems. The method is demonstrated using different examples. The measurement of the likelihood of a machine being the bottleneck aids in the decision-making regarding the allocation of the available resources.