The style.cfg shows the version of the style (and of the phpBB fileset unless update/upgrade instructions weren't followed properly), but on boards using a style other than prosilver, sometime styles that are not up to date are used (not a good idea).
Also the style.cfg only gives information pertaining to the version of that specific style, but tells nothing about the phpBB database version. For that information database access is needed.
Spelling is freeware, which means you can use it for free.
On the other hand, it is not open source, which means you cannot change it or publish it in a modified form.