What do you mean by 'more than 50% of the database'?
If you want indexes that have a logical fragmentation higher than 50%, just query sys.dm_db_index_physical_stats and filter on the logical_fragmentation_in_percent column.
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