I ordered some AT28C256 EEPROMs, but none of them were cleared, they all have some garbage programmed, probably from the last user. Obviously they're supposed to be clean and new. I wanted to ask, is there any way I can determine how many write cycles are left before the EEPROM becomes useless? I know they start with 100,000 writes from the factory, but can I figure out approximately how many writes I can have without knowing the history of the EEPROM's use? I wanted to decide whether I should return them or not. If they've been used a lot, I will. If they still have a long way to go, I'll avoid the effort of returning them.
No, there is no way to know how many times they have been programmed. A typical user will only program them a few times, so you should be OK. If you plan on programming them tens of thousands of times, you should get new parts.
EEPROMs from the factory are normally unprogrammed (all 1's).
100,000 writes is a "guarantee", they won't magically die at this number. A typical part will last much longer.
As others have indicated a typical EEPROM at room temperature can do many, many more than the guaranteed 100'000 write.
There have been many tests of these performed such as: