My bet: combination of cyclomatic complexity(CC) and code coverage from automated tests(TC).
CC | TC
2 | 0% - good anyway, cyclomatic complexity too small
10 | 70% - good
10 | 50% - could be better
10 | 20% - bad
20 | 85% - good
20 | 70% - could be better
20 | 50% - bad
...
crap4j - possible tool (for java) and concept explanation ... in search for C# friendly tool :(
