I was curious so I took a quick look. Good indicators are:
1. Most of the code is error checking/handling. This isn't necessarily a good indicator in general, but for C code it is.
2. Consistent style.
3. Functions are static unless otherwise needed.
2 and 3 seem minor, but in my experience, 2 is indicative of someone who pays attention to details and 3 suggests that the person is an experienced C coder.
