There is a number of functions in the new l3sys package for testing the
current engine, but I am still missing a public interface for testing
the driver. The l3drivers package internally uses some mechanism to
distinguish between pdfmode, dvips, dvipdfmx and xdvipdfmx. Will there be
such a testing be included in the public interface of expl3 one day?
Currently, I am still using a combination if the `ifpdf' package and the
\*_if_engine* functions for the purpose of driver detection.