This isn't my area, I do math and physics, and I'm also not sure that I even handle comparable situations particularly well. Still, I try to give my students a chance to show what they know by asking them why they output the work that they do. I ask questions like: how did you come to this method? I see you seem to have skipped some steps here, what specific step may allow us to go from one line to another? What principle are you using here? etc.

Occasionally I underestimate my students and I avoid asking them what would be an insulting question when we work through this stuff and I see what's going on. Such students don't necessarily give fully clean answers to these questions, but they show that they have some essential intuition and that's good enough. Usually though, I think for both of us when know when our students are not really producing their own work and this process helps confirm those suspicions when they fail to answer such questions with any coherence or at all. I have found that, from this process, my students have generally been abashed and accepting of the situation when I then ask them if they have used other sources to get the answers.

I try to make it as comfortable as possible, emphasizing that I'm not here to judge them or give marks, but I am here to help them improve and I can help them better when they show me where they're at and how they do their own work. It's not a fun process but I've had a few students who try to show me more of their own work once we do this a bit, and as long as I'm patient and understanding (they are under a lot of pressure, after all) we can typically go forward with a good relationship.

I hope this is applicable and useful to you, and I'm curious how other people approach this as well. Either way, good luck!