Not to say I think the Mystery Shopper offer is real, but here's how to verify a Cashier's Check if you have it in hand:

https://wallethub.com/edu/cashiers-check-scams/16192/

The Comptroller of Currency has a list of known counterfeit checks. See here:

http://www.occ.gov/news-issuances/alerts/2016/index-2016-alerts.html

The Navyfederal Credit Union doesn't seem to be on the list so if you determine fraud, you might want to submit your info to the COC.