How did Dumbledore know about the locket in the cave?

When the kids from the orphanage were taken out for some fresh air, Voldemort and a few others who were constantly threatened by him would venture into the nearby places including the caves close to the shore. One such cave is where Dumbledore found the presence of the locket. read more

Once Dumbledore had established that the cave was worth checking out, it was a simple matter (for him) to notice that the area was steeped in magic which had no business being there unless it was to protect a horcrux. read more

—Harry Potter in the cave with Albus Dumbledore searching for one of Voldemort's Horcruxes The inside of the cave Voldemort later decided to use this cave as a location for one of his Horcruxes (Salazar Slytherin's Locket). read more

However, two people are needed to retrieve the locket from the basin. Harry is underage and small, so his presence in the boat goes undetected. If Dumbledore did not know this before he actually entered the cave, it is then fortuitous that he brought Harry with him. read more