There is no real technical solution, outside of a secure facility with thorough physical inspection, isolated networks, etc. Outside of military/defense project

There is no real technical solution, outside of a secure facility with thorough physical inspection, isolated networks, etc. Outside of military/defense projects, who is going to have that? Even then, I doubt it would be effective. You can have NDA's but it will likely be difficult to discover these violations. The recruiter isn't going to rat out your ex-employee. The new company isn't, either. It'll create nothing but problems for them. This sort of stuff happens all the time. I worked at one company where we asked a potential employee to submit a code sample. He submitted some billing code he had worked on for a major telco with their copyright all over it. The truth is this doesn't really matter. Except in certain rare cases, code alone is worthless without the operational knowledge, the other parts of the system, services, databases, etc.