From pg_settings, you can see which parameters have non-default values, and what the source of the current value is.
The SHOW command doesn't tell you whether a parameter is set at a non-default value. It just tells you the value, which isn't of much help if you're trying to understand what is set and why.
If the source is a configuration file, then the sourcefile and sourceline columns are also set. These can be useful in understanding where the configuration came from.