Simulations, described in the previous paper, by nonequilibrium molecular dynamics of stress relaxation in a model polymer melt of freely jointed chains with N = 200 bonds exhibited an incipient plateau, followed by a period in which the relaxing stress corresponded to the entropic stress with an entanglement length of N-e = 40 bonds. This paper describes two types of diagnostic subroutines used to search for the entanglements or obstacles responsible for these phenomena. The first examined the rate of separation of initially neighboring interchain atom pairs but did not serve to uncover many topological entanglements. The second introduced a measure of time-averaged atomic mobility and found that intrachain atoms of relatively low mobility tended to cluster in groups along the chain. The average spacing between clusters appears to introduce a new length scale in polymer melts that is independent of the conventional picture of interchain entanglements.