The fact that the bag has not been opened will not ensure that the flour is still okay to use.  There is a window of a few months past the Best Before Date (BBD) when the product is still OK, depending on how it was stored.  The flour can be okay for a long time after the BBD if stored cold or frozen.  If you are unsure, try the hot water test.  Put a few tablespoons of flour into boiling water, stir well and sniff.  If the product is rancid, the smell will come out with the steam.