1. Straight-Line Exploration: Traverse your Atlas in a straight line to Path of exile currency reveal the maximum amount of Fog of War. This method increases the likelihood of uncovering hidden Citadels.
PATH OF EXILE 2 WIKI

2. Target Specific Biomes: Each Citadel type is associated with certain biomes:

Iron Citadels: Grass or Forest biomes
Copper Citadels: Desert biomes
Stone Citadels: Coastal areas of any biome
Focusing your exploration on these regions can increase your chances of buy POE orbs finding the desired Citadel.
PATH OF EXILE 2 WIKI