Most likely with a check box contained under the "Advanced" button. Some newer users might do this unnecessarily, though, so potentially there could be a popup that explains in what cases this should be done (linking to relevant local policies, for example on enwiki WP:SIG). Of course, there probably should be an option in the popup to suppress future warnings.

Motivation
Often with templates such as {{moved}}, {{moved from}}, etc, a signature is not needed or should not be placed.