yes, it is possible. If you deleted some files of your game folder, Steam will re-download these files. I don't know if it also re-downloads files if they were just modified. (same file name, but other filesize)
Yup, as Paul says, under properties > local files > Verify Integrity of local Game Cache
@SoTD I think it does re-download modded files if I remember correctly. I think I did mod some game's .ini file (COD Black Ops maybe), and had to set the file to 'read only' to stop steam overwriting it.