Apart from background checks / due diligence, how can you ensure a candidate will have the work ethics they claim to have?

Its very simple.. focus on Behavior based interview questions during interview. It will help you to get true picture of candidate. Also ask question that What was the most recent compliment he got from his peer or coworker or from boss. Ask for examples.. related to group tasks or peer related work. Hope thats helpful (Y) don't forget to like it :)