I recently took a temporary job in a family-run bakery shop. 
It was agreed on that I work for the first three days on probation and - if deemed helpful - could continue on a two-day-per-week basis. 
The payment for those three probation days would be half of one of the normal days. This agreement was not in written form.

After the first month (6 (3+3) days of work), I started inquiring about receiving my first paycheck, but was put off until a future day. After repeatedly asking, I finally got payed for the probation days only, but also fired because I created too much hassle to my employer. 

I was told that I would receive the remaining 3 days of payment via bank transfer at the end of the month. 

Now I am faced with several problems:

* How can I react appropriately should I not receive the remaining
payment?
* How can I warn others about this fraudulent employer, who
shows the same disregard towards his other non-family employees?
* How can I properly sanction the behavior of said employer? Is it
fair to drop negative votes on rating pages?

Some further information for you to consider:

* The bakery is run by a family and while some of them might show similar characteristics as the head of the family, the others might not deserve to face the same consequences as him (e.g. lower revenue due to bad ratings).
* There seem to be employees in their shop under even more dire conditions: foreign, not speaking the local language, has to work up to 12 hours per day, no social contacts out of work, probably illegal, desperately asked me for help right before I got fired (but not the reason for my firing)