How to recognize a passive sentence...
- check if there is a form of 'to be' used (am, are, is, was, were, been)
- is it followed by a participle (voltooid deelwoord)?
- is it followed by 'by + noun or pronoun? If not, can it be followed by 'by zombies'?
Are all answers 'yes'? Congratulations, you have a passive sentence.