[Sholmesy]

You can't. Especially at scale (every person validating the software before voting). Paper ballots with a anonymised ledger of votes placed is, in my opinion, the best method.

[Findeton]

You can audit your ballot in some systems. For example https://nvotes.com (open source software here https://github.com/agoravoting/).

You could even create your ballot offline, even by hand.

[fredley]

Paper doesn't scale well, attacks on paper are extremely difficult to scale well, which is why paper is a good system for voting.

[Sholmesy]

It scales "well enough", in that we currently do it, and pay for people to verify the results.

In Australia a lot of this work is done by volunteers from the major parties.

Edit: I agree, its difficult to scale an attack on paper :)