Using proportional fonts, but you don't need to waste the width of a char for this signal (interestingly enough in my font ellipsis is a bit wider than 3 dots since it has more space before it).
Though personally the bigger issue for me is confusion since dots do appear in regular file names, so you can't be certain whether it's truncated or not
How do you
- detect the current pixel width of a column
- get the pixel width of your field text value
- disable the default truncation? (this would also help with this Column headers name: option to remove ellipsis on truncation it ou could do this in headers)