I would normally check the blue board for entries before I take up any new client. Any established client, good or bad, is normally there. If a client is not there or has a bad rating, I do not take up the work.
That's what I usually do as well, but I did this recently and when i took the job the rating was over 4.5, but when I checked again because I didn't receive my payment, the rating was down to below 3 and the outsourcer had been banned from ProZ. It seems they hadn't paid anybody from this big project that I also worked on. So checking the BB doesn't always help.
What should I do now? The next step in Germany would be a court order, but does that also apply for a Chinese customer?
Have you tried calling them? I know it might be expensive, by I have always found that if you call somebody they pay much faster. Also, although you may not be able to do anything through the legal system, you can always threaten them that you will be taking legal actions. For me that always seems to do the trick.
Good luck and I hope you'll get your money, one way or the other.