If you have the display equivalent of the printer font on your PC, you can find out widths from that.
If the display font is TrueType, there will be a horizontal widths table; not sure about bitmap, Intellifont or Type1 fonts, but there should be something similar with these.
Most IDEs (e.g. C++ Builder, VC++, etc.) make life easier by providing methods to perform this interrogation on your behalf, for a specified text string using a specified font at a specified pointsize.