According to this question here, I need to complete a special quest in Markarth, Windhelm, Solitude and Whiterun.
Unfortunately I didn't know this when I started the thieves guild career path and now I do not remember which of those quests are done and which are still open. I completed 2 or 3 oh those, but not all of them.
Is there an easy way to find this information? Maybe someone can post the quest names and the city associated with it. I can look up those quests in my quest log to identify completed quests.