Misalignment on running machines can be identified by a series of symptoms such as high vibration levels, coupling wear, oil leakage and abnormal machine temperatures. When you recoginze any of this you can suspect misalignment being the root cause. As a thumb of rule shaft alignment should be carried out every time a coupled machine’s hold-down bolts has been lossened.
New installations (check out this blog post)
When a machine has been on service/repair
When auxillary equipment has been attached (such as piping to a pump)
After coupling replacement
..and of course if you recognize any of the above symptoms