In the context of the quote it's usually pretty clear. If there's been some disaster you will know (or easily be able to find out) whether the drive that the log is on is intact or not.
Other than that, does a log backup succeed? If yes, the log file's intact and undamaged.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability