I am watching monster right now and in Episode 5, soon after the murder of Junkers, Tenma is shown investigating Johan's past using the name Micheal Reichmann. I seem to have missed where he got that name and how did he know where to go and do the investigation.
I haven't watched the anime, so I will refer to the manga.
In chapter 6 Mr. Junkers is hit by a car. Detective Runge mentiones that Junkers is a suspect in a series of murders. In chapter 7 Runge explains that there were four cases in whole Germany where childless couples were murdered.
When Tenma meets Johann and Junkers in chapter 8, Junkers explains that Johann hired him to help murder the four couples. Tenma tries to stop Johann, who wants to shoot Junkers, by saying that he knows Johann from when he was his patient. But Johann replies that noone knows him, neither the Lieberts (Johann was the foster child of the Lieberts when he was in hospital) nor the four couples.
By this statement you can draw the conclusion that Johann was also the foster child of the four childless couples he wanted to be killed. So in chapter 9 he probably just travelled to the places where those murders took place, one of them being Colonge where the murdered couple were the Reichmanns.