Molecular dynamics simulations were performed for a polymer melt. In quiescent states, the inter-chain interaction energy supported by each particle takes relatively large values persistently for long times if the particle is close to an entanglement. Thus, if the interaction is averaged over appropriate time intervals, we can detect active spots on the chains produced by entanglement constrains. If rapid shearing is applied, these active regions subsequently become bended in eventual zig-zag shapes of the chains. We also demonstrate that stress overshoot occurs with onset of disentanglement.