Who is responsible for checking the legality of initiatives?

The Standing Orders of the Sejm of the Republic ofPoland of 30 July 1992, Article 34.8

http://libr.sejm.gov.pl/oide/en/images/files/dokumenty/standing_orders_2009.pdf (in English - the Parliament’s website)

Law on Nationwide Referendum adopted on 14 March 2003, Article 63

http://pkw.gov.pl/g2/i/00/8.pdf (in English - but not updated)

http://isap.sejm.gov.pl/DetailsServlet?id=WDU20030570507 (in Polish – the Parliament’s website)