ISAM-A supports an additional ISMUTL operation called VERIFY. It is equivalent to REBUILD, except it only checks the index without doing any updates on it. This is somewhat faster than REBUILD, and also lets you find out whether there was a problem in the first place. (REBUILD just rebuilds new indexes from scratch, so you have no idea whether the operation was necessary or not.)

Beginning in A-Shell build 1164 of 2 Nov 09, ISMUTIL VERIFY operation outputs information to ashlog.log on errors. On the screen, it only shows the type of error without any helpful details.