Here's a copy of the manual:
http://www.tamiya.com/english/rc/rcm...58395_db01.pdf
Check out page 5.
This picture is from that page and shows you how to test each diff for slipping:
diff_test.jpg
But if you did not use diff grease, just disassemble each diff, degrease, check for wear and rebuild. The more you test, the more you will wear them out.