How to Avoid the Passive Voice
I found this on the Aerogramme Studio site. To test a sentence to see whether or not it is cast in passive voice, try adding “by zombies.” If it can be added, then you know it’s passive.
The sentence was written on the whiteboard … by zombies.
The post was added that evening … by zombies.
Not that some passive constructions aren’t okay, but still–
…to secure these rights, governments are instituted among men … by zombies …
You get the idea. 
This is absolutely delightful–and useful! I wish I’d had this tag phrase to suggest when I was teaching English 100.