At the moment, medical practitioners in Germany can only be corrupt when employed by someone else. As long as they practice on their own, German law to date knows no way of finding them corrupt – even if pharmaceutical companies give something in exchange for the practioniners prescribing certain medication.
The good news is that German health minister Daniel Bahr is about to change that.
Update Nov 2014: The bad news is, that the bill did not pass the Bundesrat. And coverage is pretty much non-existent since then.