You generally can’t fully ensure non-commercial use, but you can make the permission clear and easier to enforce.

Best practice is to give permission in writing and state exactly what is allowed, for example: non-commercial use only, what type of print/use is permitted, whether attribution is required, and that no commercial use is allowed without separate permission. A signed agreement is stronger evidence than a verbal understanding, but even email or other written communication can help show the terms.

The main value of a written agreement is avoiding disputes about what was said. If someone later uses the image commercially, the issue is less about whether your terms were clear and more about whether you’re willing and able to enforce them. That can be difficult, especially across countries.

So: use a simple written license or electronically signed agreement, keep the terms specific, and understand that it reduces risk but does not guarantee compliance.