[HALtheWise]

One reason you might not want someone outsourcing their work is because they (presumably) won't be able to explain or improve it. Consider testing that? For example, get them on a video call to answer questions about a code review, or do collaborative design work related to their project. You can apply similar practices to what an interviewer would, asking progressively more detailed questions to find out if they really did the work, although you obviously want to present yourself as an interested coworker rather than an interviewer.

At some point, getting your manager and/or their manager involved will be inevitable, and this is exactly the sort of problem that managers and HR exist to deal with. (assuming you're not their manager yourself).