If you're using source control, it's easy to track source and control access — or revoke access — if someone leaves the company.

Also, your developers should have signed a contract about the project itself that clearly places the source code under your/the client's/the company's control, and if/when they leave, they release all rights to code created under that contract.

You can never be 100% sure, but these small steps will at least ensure a legal high ground should anyone steal code.

Good luck!