I have often heard that vote rigging is detectable on HN because the site software penalizes voting from accounts at the same IP address.

Rumor had it that there is also some kind of social-network metric detecting when socially adjacent accounts (or alts) are engaged in astroturfing, the practice where a small cabal tries to pass themselves off as a broader grassroots campaign.

Flip that around though and the same metrics might allow new accounts to be meaningfully vouched for by existing ones.