How to tell which commit a tag points to in Git?

One way to do this would be with git rev-list. The following will output the commit to which a tag points:

$ git rev-list -n 1 $TAG

You could add it as an alias in ~/.gitconfig if you use it a lot:

[alias]
  tagcommit = rev-list -n 1

And then call it with:

$ git tagcommit $TAG

Possible pitfall: if you have a local checkout or a branch of the same tag name, this solution might get you "warning: refname 'myTag' is ambiguous". In that case, try increasing specificity, e.g.:

$ git rev-list -n 1 tags/$TAG

git show-ref --tags

For example, git show-ref --abbrev=7 --tags will show you something like the following:

f727215 refs/tags/v2.16.0
56072ac refs/tags/v2.17.0
b670805 refs/tags/v2.17.1
250ed01 refs/tags/v2.17.2

Just use git show <tag>

However, it also dumps commit diffs. To omit those diffs, use git log -1 <tag>. (Thanks to @DolphinDream and @demisx !)