Right now deepfakes doesn’t work well when the face is viewed from extreme angles, so you can ask them to slowly turn their face to the side or up/down as far as they can until the face is not visible. It also doesn’t work well when something obstruct the face, so ask them to put their hand in their face. It also can’t seem to render mouth right if you open it too wide, or stick out your tongue.
I base this from a deepfake app I tried: github.com/s0md3v/roop . But as the tech improves, it might be able to handle those cases in the future.
Edit: chance that the scammer use a live deepfake app like this one: github.com/iperov/DeepFaceLive . It also supports using the Insight model which only need a single well lit photo to impersonate someone.