Before checking it in again, run rcsdiff yourfile . The RCS system will show you what's different between the working version and the most recently checked-in version of your file. After you've made a couple changes, check it in again as in step 2. You'll be prompted to describe briefly what you did to the file. The file's version number will be incremented and your write access will be taken away until you check the file out again.
Emacs fans can check files in and out of RCS without shelling out to the command line.