Find out when your index was last rebuilt / reorganized
SQL Server does not maintain when an Index was last rebuild, instead it keeps information when stats were last updated. 

That can be found using the STATS_DATE function.

You can use Ola's Index maintenance solution or Michelle Ufford's - Index Defrag Script. These scripts are widely tested in the community and are much flexible so that you can adapt as per your needs in your environment.

SQL Server  SP2 for 2008R2 and up has a new DMF sys.dm_db_stats_properties which tells you when your stats were last updated with other info like 


  modification_counter: number of modifications for the column which leads the statistic, since the last update