If you ordered "straight run", they will be probably be unsexed. If you specifically ordered males and then another order of pullets, they should be marked.
If you don't see any marks telling you which sex they are, you will be able to tell the males by around five weeks. Their Wyandotte feathering will be much more vivid and contrasting than the pullets which will appear more drab in comparison. The boys combs will also be a little more pronounced and redder than the girls. By age six weeks, you will know for sure which are cockerels and which are the pullets.