Acceptable documents for German citizens
The primary documents which are used for proving identities are the identity card ("Personalausweis") and the passport. Since every German is obliged to own an identity card or a passport from the age of 16 on you should insist that at least one of the presented documents is an id card or a passport.
A driver's license usually also is accepted but they do not have an expiry date and so you may be presented a very old (and easily forged) license. That's why I'd consider it a secondary document.
A birth certificate does not contain verifiable personal attributes, but since they are only handed out to the person in question I'd accept them as a secondary document.
Other secondary documents could be company badges, account cards of public transport authorities and the like. You should only accept those if they contain a photo of the person, you have seen those before and you have reason to assume that they are not easily forged.
Note on birth names in Germany
Since the late seventies it has been possible for a man to adopt (or prepend/append to their birth name) the woman's name at marriage, so both sexes can have a birth name on the id card.
Since early 2001 same-sex unions became officially possible, and one of the partners may adopt the common name or prepend/append it to her/his birth name (§ 3 LPartG).
There are two formats of valid German id cards.
Since November 2010 the German id card has the size of a credit card and is of greenish colour.
The front side contains a colored photo, name (possibly including birth name), date and place of birth, the nationality and the expiry date. There also is a facsimile of the person's signature. There are several holographic images, among those the Coat of Arms of Germany, the flag of Europe, the name, the id number and the photo.
The backside contains the current address, height, color of eyes, a possible religious name or pseudonym, the issuing authority, the date of issuing and three lines of "machine readable data". The name of the person and the passport number can be seen both in the machine readable lines and in a horizontal silver stripe. The backside also bears holographic images: Another instance of the photo and the expiry date. Please see here for details.
The German id card issued from April 1997 to October 2010 has a size of about 7x10 cm, is of greenish colour with a more reddish stripe on the backside. It is laminated in plastic, the front side of the plastic contains engraved structures (stylised eagles).
The front side contains a colored photo, name (possibly including birth name), date and place of birth, the nationality and the expiry date. There also is a facsimile of the person's signature and two lines of "machine readable data" (nationality, name, card no, birth date and expiry date with some checksum digits in an OCR font).
The backside contains the current address (which may be on an adhesive label if the person moved since the card was issued), height, color of eyes, a possible religious name or pseudonym, the issuing authority, the date of issuing and once again the name of the person.
Newer versions since November 2001 of the ID card have holographic images embedded on the front side.
An id card is valid for up to ten years (generally five years for youths) from the date of issue. So any German id without holographic images has expired.
Link to detailed description of the German id card.
The first (inside) page of a passport is a laminated card quite similar to the front side of the id card, but it is slightly larger (about 12x8.5 cm) and contains the issuing authority, date of issue on the front side. The backside of the laminated card contains no personal attributes.
The German printing factory (Bundesdruckerei) has an archive of official German identity documents.
The rest of the passport won't help you very much for the purpose of assurance and I'd consider it quite rude to browse the visas without explicitly asking. (See also Arbitration Case a20140204.1).
Should you ever have to note the passport id, please do not get trapped by the weird typography employed - instead, consider Typography on German passports first.
To my knowledge there are currently 3 kinds of driver's licence in use.
The oldest kind has been issued from at least 1959 till at least 1985. It consists of quite thick bluish grey paper with a size of 20x14.5 cm. It contains the current name (which may be the birth name or a married name), date and place of birth, residence (which will most probably be incorrect since the document is at least 20 years old), the issuing authority, a number/letter code, the owner's photo and signature (remember, they are at least 20 years old!).
From the view of forgery proof this document is quite useless, the only distinctive thing is it's paper. Nevertheless it is usually accepted as a proof of identity. I really would not assure a person who only presents this kind of driver's license and another questionable document.
At some time between 1986 and 1998 the second kind of driver's license has been issued. It is also made from thick paper, but has a size of 21x10.5 cm and is of reddish/pink color with a pattern of fine lines on the background. It contains about the same attributes as the first kind of license and has some security features. Has a marginally improved resistance against forgery.
Since 1999 the EC driver's license is issued in Germany. It consists of a credit card sized bluish-red plastic card with several holographic images embedded on the front side. To me this one looks like it is at least as forgery proof as a current ID card. For a detailed description see marking details (one security feature on the backside is missed!) so at least 14 security features you can check.
See also the wikipedia article.
See also PRADO database Germany documents: http://www.consilium.europa.eu/prado/DE/categories/showAllCategories_DEU.html