Clarifying your needs is the key to hiring offshore developers. The offshore developer must possess the requisite programming languages and state the frameworks clearly. For instance, If you cannot handle the front-end part of the product with your in-house team, you can search for offshore developers with a better grasp of technologies like HTML, CSS, Javascript, etc.
For your reassurance, you must conduct a formal interview and require them to pass a demanding technical test. If you're interested in hiring a Backend offshore developer, you've to create a test that lets us know their abilities in different criteria. You must also take cultural compatibility into account.