1) get the indexes defined over the table first by using the foll query:
SELECT * FROM SYSIBM.SYSINDEXES WHERE TBNAME='TABLE_NAME'
2) by executing the following query you will come to know what are all the keys present in each indexes and the common column in all these indexes will be the primary key.
SELECT * FROM SYSIBM.SYSKEYS WHERE IXNAME='INDEX_NAME'
You can also find the primary keys for any table using the PLATINUM of DB2 sub menu.