How force a class to get a UnitTest for every public method?

we are starting with UnitTests now to raise our code quality a lot and (of course) for different other reasons. Sadly we are very late to the party, so we have classes with ~50 methods which are completly untested until now. And we have a lot of classes!

Is it possible / how is it possible to:
a) see which methods are untested?
b) force a developer who adds a method to a class to write a unit test for it (e.g. I achieve to test all 50 methods and tomorrow someone adds a 51. method)?
c) raise a warning or a error when a method is untested?