In general, a player sitting over dummy, and unsure which suit to lead, should lead up to dummy’s weakness in the hope or knowledge of leading up to partner’s honours in the suit.
For example (with North as the dummy):
East, on lead with no definitive information to guide him, should lead a heart. Any honours West has in hearts will be useful over South’s holding whereas it will be hard work to establish a spade trick in view of North’s strong spades.