Australia has done this for a century. It's not that hard. You have official counters and each candidate can appoint scrutineers to observe the count[0]. Scrutineers can challenge how a ballot has been counted: "I think that's actually a 7, not a 1". If the count supervisor agrees, it's changed. If not, then not. If it's ruled impossible to distinguish, the ballot is considered informal and removed from the count entirely.
[0] https://www.abc.net.au/cm/rimage/7570634-3x2-xlarge.jpg?v=6