[sitharus]

Through process verification.

I don't know about other countries, but here in New Zealand elections are monitored by the Electoral Commission who employ people to watch every step of the process. In addition every political party is entitled to send scrutineers to polling places and the count. The boxes are shown to the scrutineers who verify it to be empty, then sealed. The boxes are never taken out of sight. Every step of the process is under multiple eyes.

This seems to work well. It's possible to double vote by visiting multiple stations but in practice this is rare. It does happen but the electoral commission do analysis on the votes to ensure it didn't affect the outcome. Those responsible are prosecuted.