How do I check for render errors in the logs?

If you created a proxy and are using the right renderer, Golaem Crowd should output some errors or warnings in the script editor if something goes wrong,.
 
Just open it and check if Golaem Crowd reported something after you clicked on render.
 
 
Note that errors are often not outputed in the script editor but in the Output Window (because of the way some renderers works).