Generally speaking, the capital letter M is the widest letter of a font. Another letter that is often as wide, or almost as wide, is the capital W. If you want to know for sure, print them out and measure.
Print something using your font. Make sure it contains a capital M and a capital W. If it is a dingbat font, you might have to print out an example of each character. Make the font at least 24 points.
Measure the width of each letter to see which one is the widest. You will probably be able to visually compare and narrow it down to just one or two letters that are obviously the widest and only measure these to be sure which one is the actual widest. Capital letters "M" and "W" are typically the widest letters in most fonts.
If you are unsure, print the letters in a larger font and measure them again. Use a ruler that shows at least 1/8th inch marks, and 1/16th inch marks if possible. Or use a pica ruler that shows picas and points. A pica is 1/6 of an inch, and there are 12 points in a pica.
Fonts enlarge proportionately, so if you know what the widest character is when the font is 24 points tall, it will be the same even if you enlarge the font to 100 points.