Verity Newman is related to Joan.
Martha and Mickey- He seems to aware of alien activity on Earth, seeing as there was a Sontaran after them, he found them as well
Sarah Jane and Luke- He went to Bannerman Road already in The Wedding of Sarah Jane Smith
Captain Jack- That's a tough one, erm a shot in the dark is he went back to see Gwen and she told him about Jack going onto space on a freighter and the Doctor tracked him down
Verity Newman- A book signing about him wouldn't be too difficult to find also the reason he went to Verity instead of Joan is because he didn't want to hurt Joan any more than she had been and open old wounds
Donna, Wilf and Sylvia- He went to a church in Chiswik closest to Donna and founfd her there, he could have looked in the newspaper for the wedding details.
Rose- He's been to the Powell estate many times
In The Sarah Jane Adventures story The Death of the Doctor, he tells Jo that she was moving around so much "even the TARDIS" couldn't track her down. That implies he used the TARDIS to find the others, with the possible exception of Sarah Jane, whose home he had already visited once. As pointed out above, he would avoid Joan because of the hurt he might cause her. Instead, he visited her greatgranddaughter Verity to find out how Joan had fared, showing that, as the Doctor (not just as John Smith), he cared about her very much indeed.