A common workflow is to sed -i '<expression> somefile.conf to change things in the file, followed by cat somefile.conf to see if things changed.
Using word designators in Bash is pretty useful for this. e.g. In the scenario above, after the sed command running cat !$ will show you the (hopefully, correctly modified) file contents.
From this answer, to show the sections of the bash manpage on history expansion:
man -P 'less -p ^HISTORY\ EXPANSION' bash