Is there any way in Windows or Microsoft office to tell beforehand if a font is fixed-width, other than trial and error?
If you click on the drop down arrow next to the font name display box in Word you should see a listing of all your fonts’ names in that font. This makes it very easy to see which are fixed-width.
And almost none of them are. The only one I have is Courier (and its variant Courier New). Most editors still retain a preference for Courier font in a manuscript, just because it makes it so much easier for them to estimate the length of a manuscript something mere word count can’t do. Prestige Elite and Monaco are other such fonts.
But except for those and a very few others, you’d probably have to search for a fixed-width font to download.