But before you go off the deep end, 0xff (or -1 for signed character types) are also legal values. Even worse, if someone used the EEPROM before with another Program, the EEPROM can contain data which are junk for the current one. So don't bother checking for 0xff, it will give you only heartache and pain.
If you want to know if the EEPROM has been already written your current application, write a magic marker (eg the integer 6934 at the beginning and a CRC or checksum at the end of your data. If both are present and correct, your chances are greater that that nobody messed with the data, although it's not perfect.
Korman