As blue112 noted, the dividing line is fuzzy. The very first documentation page, however, has an explicit list (and as R.M. notes below, one primary criterion is, or at least is supposed to be, stability of interface—for which some nominally-porcelain commands have --porcelain1 to force a more stable and/or more machine-readable output). You can choose to use their list, or decide that some commands are too high level to be low level, or too low level to be high level. For instance you might disagree that git apply is a plumbing command, yet the Git page says it is. Or, you might consider things like git fast-import to be something you would only use inside a script.
Edit, 31 May 2020: The Git documentation has changed since 2016 to reassign commands to new sections; the data below are now out of date. Consult the documentation for your own Git version (via git help git for instance) to see what your own system says. See also VonC's answer with a link to a 2018 change to the documentation.
The list below is simply extracted from the documentation, with descriptions and additional classifications stripped to leave only "porcelain" vs "plumbing". (Sub-classifications remain visible as inversions in the alphabetic sort order. I did not construct links for each entry, as this would be considerably more difficult with StackOverflow markdown—this just needed a simple <pre>...</pre> wrapper.)
porcelain
git-add git-rebase git-cherry
git-am git-reset git-count-objects
git-archive git-revert git-difftool
git-bisect git-rm git-fsck
git-branch git-shortlog git-get-tar-commit-id
git-bundle git-show git-help
git-checkout git-stash git-instaweb
git-cherry-pick git-status git-merge-tree
git-citool git-submodule git-rerere
git-clean git-tag git-rev-parse
git-clone git-worktree git-show-branch
git-commit gitk git-verify-commit
git-describe git-config git-verify-tag
git-diff git-fast-export git-whatchanged
git-fetch git-fast-import gitweb
git-format-patch git-filter-branch git-archimport
git-gc git-mergetool git-cvsexportcommit
git-grep git-pack-refs git-cvsimport
git-gui git-prune git-cvsserver
git-init git-reflog git-imap-send
git-log git-relink git-p4
git-merge git-remote git-quiltimport
git-mv git-repack git-request-pull
git-notes git-replace git-send-email
git-pull git-annotate git-svn
git-push git-blame
plumbing
git-apply git-for-each-ref git-receive-pack
git-checkout-index git-ls-files git-shell
git-commit-tree git-ls-remote git-upload-archive
git-hash-object git-ls-tree git-upload-pack
git-index-pack git-merge-base git-check-attr
git-merge-file git-name-rev git-check-ignore
git-merge-index git-pack-redundant git-check-mailmap
git-mktag git-rev-list git-check-ref-format
git-mktree git-show-index git-column
git-pack-objects git-show-ref git-credential
git-prune-packed git-unpack-file git-credential-cache
git-read-tree git-var git-credential-store
git-symbolic-ref git-verify-pack git-fmt-merge-msg
git-unpack-objects git-daemon git-interpret-trailers
git-update-index git-fetch-pack git-mailinfo
git-update-ref git-http-backend git-mailsplit
git-write-tree git-send-pack git-merge-one-file
git-cat-file git-update-server-info git-patch-id
git-diff-files git-http-fetch git-sh-i18n
git-diff-index git-http-push git-sh-setup
git-diff-tree git-parse-remote git-stripspace
1It would seem more logical to call this --plumbing, but as VonC notes in this answer to a related question, one can view this instead as a request: "I am implementing porcelain so please give me plumbing-style output." The flaw in this argument is that you might be implementing complex plumbing, and want to use simple plumbing to do it: now there's no porcelain in sight, and yet, your complex plumbing passes --porcelain to some simple plumbing.
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