Answer: with some difficulty!
Visit
for information about the OpenType specification (a superset of TrueType) and you'll begin to see that extracting character metrics from a TrueType font is not a trivial task.
The same is true of PCL fonts, which come in a number of variants, within 3 'groups':
(a) PCL bitmap fonts.
(b) PCL Encapsulated Intellifont Outline (PCLEO) fonts.
Downloadable files usually have .sfs or .sfd extensions.
Not used much these days; Intellifont specification is difficult, if not impossible, to get hold of.
(c) PCL Encapsulated TrueType Outline (PCLETTO) fonts.
Downloadable files usually have .sft extension.
The above typography site may have links to tools which may provide font metrics.
Otherwise, consider commercial tools such as FontLab, or the (much cheaper but almost as good?) FontCreator package from High-Logic.