You can check the block chain, all the POW block is mined in the only pool, so, I don't think this method works.
I checked more into the block chain - went over 100~ blocks , these are the PoW blocks i found
Block Height Block Time Time Since Last Block
99321 January 14, 2014, 04:26:19
99313 January 14, 2014, 04:22:15 00:04
99296 January 14, 2014, 04:15:27 00:06
99289 January 14, 2014, 04:10:58 00:04
99263 January 14, 2014, 04:01:17 00:09
99249 January 14, 2014, 03:55:23 00:05
99247 January 14, 2014, 03:54:20 00:01
99230 January 14, 2014, 03:41:39 00:12
the average looks fine for the x12 rule (6 mins)
I checked further
Block Height Block Time Time Since Last Block
99321 January 14, 2014, 04:26:19
99313 January 14, 2014, 04:22:15 00:04
99296 January 14, 2014, 04:15:27 00:06
99289 January 14, 2014, 04:10:58 00:04
99263 January 14, 2014, 04:01:17 00:09
99249 January 14, 2014, 03:55:23 00:05
99247 January 14, 2014, 03:54:20 00:01
99230 January 14, 2014, 03:41:39 00:12
99214 January 14, 2014, 03:36:59 00:04
99201 January 14, 2014, 03:30:16 00:06
99184 January 14, 2014, 03:18:30 00:11
99166 January 14, 2014, 03:11:51 00:06
99164 January 14, 2014, 03:11:25 00:00
99105 January 14, 2014, 02:42:30 00:28
and it seems like between 99164 and 99105 there are only PoS - and it's way more than x12 block time
I went over all the blocks in the block crawler twice between them
I am identifying PoW by their difficulty because it doesn't say "PoW/PoS" there
Plus - is there a way to see more than one block at a time in the block crawler ?