I have a ceramic knife that I did not use for a long time and I am not sure if it was fleshing or not, can I kasher it?
Although in general ceramic cannot be kashered, in this case where you do not know if it is fleishig, the halacha is different.
You should do hagaala on it three times, i.e. clean it well and then insert it in boiling water 3 times. It is preferable to use different boiling water each time. Immediately afterwards you should immerse it in cold water or pour cold water on it.
All the best