You can tell "rufus" is a loan because it contains medial -f-, and native Latin words don't contain medial -f-, except after prefixes. The only Latin sources for /f/ are Proto-Indo-European bh dh *gʷh in initial position; elsewhere, they generally become /b d v/, in a process that probably paralleled the rhotacism.

This sound change wasn't shared by Oscan, Umbrian, or Faliscan -- compare Faliscan "carefo" to Latin "carebo", and Oscan "mefiaí" to Latin "mediae".