Forfeiture statutes differ from jurisdiction to jurisdiction. In my state, there has to be a conviction first. In the federal system (and maybe some states) you don't have to have a criminal conviction. The government just has to prove it was used in a crime or was the fruit of a crime. The administrative forfeiture provisions discussed in this thread allow the trial to be conducted by an administrative hearing officer, not a jury or full fledged judge (or even a magistrate judge). The party can appeal.
I may be wrong but I believe the standard of proof in a civil forfeiture proceeding is clear and convincing evidence. That's more than a preponderance (most civil cases) and less than beyond a reasonable doubt (criminal).