If you want to do manual inspection on the file names, then use the -i/--ignored flag to status:
$ hg status -i
I ignored file.exe
If you want the file names alone, then use -n/--no-status to suppress the I status code printed in front of each filename:
$ hg status -n -i
ignored file.exe
If you need to process the files with xargs, then use the -0/--print0 flag in addition:
$ hg status -n -0 | xargs -0 touch
That will take care of handling spaces correctly — with using -0, there is a risk that you'll end up treating ignored file.exe as two files: ignored and file.exe since shells normally split on spaces.
The above commands show you untracked files matching .hgignore. If you want to solve the related problem of finding tracked files matching .hgignore, then you need to use a fileset query. That looks like this:
$ hg locate "set:hgignore()"
You can use filesets with all commands that operate on files, so you can for example do:
$ hg forget "set:hgignore()"
to schedule the files found for removal (with a copy left behind in your working copy).