Hmm.. for distinguishing senior engineers from juniors I would look beyond just knowledge on technical topics. Two important areas to cover IMO for senior candidates, beyond coding skills:

1. Design / architecture / product skills. Here I mean 'design' in the software sense: can you design a good API, can you structure a large project well? Do you have good enough product intuition to work well with a product manager and a designer?

2. Past experience. What have they shipped in the past? How did they deal with difficulties like conflict between teams, deadline misses, uncooperative team members, etc.