I misplaced my manual, and I don't know how to adjust the valves. How often to you adjust them? What are the signs that they need to be adjusted?

 

Were they checked at 10 or 20 hours? Thats when the first check should be done. If it wasnt done then, it should be done now.
Usually the symptoms are noise if they get too loose, and hard starting when they get tight. If the valves tighten up, and you dont catch it, the valves wont cool properly (because they dont contact the seat to transferr heat), and can 'burn'...or erode, and then they need to be replaced, instead of a simple adjustment.
Heres a good how to
Here