Template:\sandbox/doc

Returns "/sandbox" if the template detects that the page where it is transcluded is a subpage named "sandbox" (i.e. a sandbox page); otherwise it returns nothing.

This can be useful to e.g. switch the version of a template used in its /doc[umentation] to the version in its sandbox (by replacing  ... on the /doc page with   ...) when that documentation is transcluded on the template's sandbox page.