Anyone know of a good way (website?) to determine someone's postal code/postleitzahl from a street address?

Just from a street address you will not be able to figure the PLZ (Post
Code). You will need the city/town name to get the code.

Try the Post website for some assistance. On the right, type the
town name under "Postcode" and click on "start".

Sorry, I wasn't clear - I have the street address AND town/city name.

For example, Zurich has many postleitzahls.

But the Post link works perfectly, thanks!!