What is a reliable method to locate floating islands in Terraria?

I've found that using several gravitation potions is the most efficient method to locate floating islands.

• It clears the black unexplored territory of the sky on your map
• You know for a fact you did not miss any floating islands
• It only requires a few gravitation potions for the entire world (depending on size)
• You do not have to wait for projectiles to bounce back

Really the only downside to them is that they are rather difficult to obtain early game, as you can only find them in pots and chests until you are able to obtain fireblossom and feathers.

One of the best and quickest methods (apart from external map viewers) is to use Meteor Shot or Water Bolt. Just keep walking on the ground and shooting bolts straight up, if they bounce back there is an island above you.

More information and other good tips can be found here.