NS bureaucracy is not my expertise, this is just what I found in a quick research. If anyone with expertise wants to expand or correct, I'd appreciate it.

For simple citizens the administrative clerk could exercise discretion and consider a written statement of "non-Jewish" heritage to be sufficient. However, if the clerk had doubts about the heritage or if the person in question had applied for a position as a civil servant or any other form of high political or administrative office, it was necessary to provide proof of the last five generations. If those could not be provided, be it because the applicant was an orphan, born out of wedlock or simply because the government files could not be found, an assessment through the "Sachverständigen für Rassenforschung" (experts for race investigation, my translation) was necessary. This was a subdivision in the Ministry of the Interior that could on the one hand help with finding lost government files but on the other hand also had "race experts" that would analyze a person's "race characteristics" to establish if he or she had "Jewish characteristics".