I usually find them by accident when I am looking at a descendent family tree. Otherwise for me, it is a hit and miss thing.
BUT if you are creating a new profile and you get a list of possible duplicates showing up, then you MUST check that list to ensure that you are not creating a duplicate.
If you do find a profile with all the right details already on wikitree, then you can cancel the one you are making and add your new sources and details to the one that is already in the system.