In general, a player sitting in front of dummy and unsure which suit to lead, should lead through dummy’s stronger holding, in the hope or knowledge of leading up to partner’s honours in the suit.
For example (with North as the dummy):
West, on lead with no definitive information to guide him, should lead a spade.
If, for example, East has K-J-x in each major, the king-jack will be well placed in spades, over North’s ace-queen, but useless in hearts under South’s holding.