I have found out how to find all dependencies on a table.
However if I just wanted to know where this primary key is used, how would I do it?
I believe we use SQL Server 2005.
I assume you're looking for all foreign keys that are based on a given table (and it's primary key). If you were on SQL Server 2008+ you could use sp_fkeys.
EXEC sp_fkeys 'TableName'
As you're on 2005 you'll have to put together this data manually. Check this previous answer by Gishu, it should give you the script you're after.