Find missing indexes
SQL Server renders a handful of DVMs that can provide you with this information.  See the TechNet reference on About the Missing Indexes Feature.

Mainly, you will be dealing with four DMVs:


sys.dm_db_missing_index_group_stats
sys.dm_db_missing_index_groups
sys.dm_db_missing_index_details
sys.dm_db_missing_index_columns


Note: This is how you do it.  But be very careful when considering "missing indexes".  You could end up with a large amount of indexes and data manipulation will take a hit from that.  This does not replace prudent index consideration/creation.