Each client decides for itself which nodes are more trustable. The algorithm can be different, it's up to the client. At the moment we use Proof-of-Work: each node demonstrates a Score calculated. The client compares the Scores and picks nodes with the largest values. In the future we will use Proof-of Availability, where clients will put Scores on nodes, depending on their availability in the network, like how far they are, how fast they answer, how many times they respond with 404-error when asking them to return wallet data.