How can one kasher a crystal decanter that likely held wine? Hagola may crack it.
If it wasn’t washed together with treife keilim, it does not have to be kashered.
יו״ד קלה, ח. וגם הרמ״א לא הגי׳ – ראה מג״א תנא, מט. ובמנ”י כלל פה סקי”ב כתב להיפך דהוא משום חומרא דחמץ.
Q. I do not know if it was washed with other keilim. It was purchased used.
A. I cannot imagine it would be washed in a dishwasher. It normally would be washed by itself.
For extra measure, you can do “milui virui”: It should be filled up with (ambient) water and simultaneously submerged in water for 24 hours. That water should then be emptied and it should repeated for another 24 hours, then emptied and then repeated for another 24 hours.