From memory, it's either this
SELECT * FROM sys.objects
WHERE type = 'PK'
AND object_id = OBJECT_ID ('tableName')
or this..
SELECT * FROM sys.objects
WHERE type = 'PK'
AND parent_object_id = OBJECT_ID ('tableName')
I think one of them should probably work depending on how the data is stored but I am afraid I have no access to SQL to actually verify the same.