EEPROM wear levelling

This part is unclear. 0xFFFF is decimal -1 when interpreted as an int16_t, and -1 is within the range of my values. How would I know that 0xFFFF is anything special as opposed to a legit value?

I use EEPROM.read and EEPROM.write and do all the conversions manually. Of course, I only update the EEPROM when necessary, that is, when I must update the value.