I've found that the behavior of "the little dot in the center of the magnifying glass" is inconsistent as a means for determining whether or not Spotlight indexing is going on.
A more reliable way is to launch Activity Monitor and see whether the mds and/or mdworker processes are active and responsible for a significant share of ongoing CPU usage. (That amount will vary depending on your system; roughy half of my mini's single 1.25 GHz G4 processor cacpacity is typically occupied by these two processes combined when reindexing is going on.)
