VC grabs a bit of the mode line for each buffer visiting a registered file and tries to use it to keep you informed of the version control state of that file. You'll notice that when a buffer is visiting a version-controlled file, the mode tags part of the mode line (shown in parentheses) shows the name of your version control system and a revision number for the file.
When those two parts are separated by a dash, the file is not yet checked out; when they're separated by a colon, the file has been checked out, and the revision number is the one the file had when you checked it out. Note that since most people use concurrent version control systems these days, in which you don't check files out or obtain locks, you can think of the dash as meaning unmodified, while the colon means there have been changes that are not yet committed to the repository.
If you don't see these indicators, the file isn't registered yet. These three states are illustrated in Figure 12-3.
Figure 12-3. Mode lines showing a file that is not under version control, one that is unchanged with respect to the repository, and one that has had changes saved but not yet committed.