To see if teachers have posted their grades, you can run the "Grade Post Verification" report in the Grades top tab, Grade Input side tab.
The inputs will look something like this, depending on the term, and it will show you the classes for which people have not yet clicked "Post Grades". It won't show if there are missing grades among the posted grades though, just if teachers have clicked "Post Grades". 
Depending on your school level, you'll probably see some sections like homeroom, student progress, or MCAS etc as unposted, but those sections don't post report card grades and are just for roster purposes.