In terms of error detection, one of the principal tools in double entry is the Trial Balance. A trial balance lists all of your accounts in your ledger, with debits and credits in opposing columns, sums them all, and the two columns totals should match. If they do then all entries are at least complete. If they don't then you have lost records along the way. By correctly assigning debit or credit normal to each account type you ensure there is always a credit for every debit and vice versa.