This paper provides a novel method for detecting bottlenecks in manufacturing systems and the shifting of these bottlenecks. All manufacturing systems are constrained by one or more bottlenecks. Improving the bottleneck will improve the whole 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 shifting bottleneck detection 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. This allows the use of simulation to predict bottlenecks for both steady state and variable systems. The measurement of the likelihood of a machine being the bottleneck aids in the decision-making regarding the allocation of the available resources.