Alright people here come big spoilers from the book so if you want to keep the surprise, do not read further.
The Titans are born in planets. Their spirits are named "world-souls" . When they finish their growth, they awaken. It is not exactly sure how they are physically because they are described as "living worlds", with "skin crisscrossed with mountains and oceans", so that would mean that they still are planets but here's a depiction of them and another one in which they resemble huge cloudy nebulae.
To find them and determine if a planet was a world-soul, they would use certain techniques :
* First, they would calm the Elemental spirits roaming across the surface
* Finally, they would plant a ton of different life forms across the world
But even then, they vowed to protect the worlds they visited. They have their own methods :
* They would imbue primitive life-forms with huge amount of powers so they could protect the world
* And plant colossal machines that would allow them to monitor the world, or purge it of corruption
The only issue with all that, is that the Titans were not the only ones loonking for world-souls...