How I'd do it:

1. Define reasoning (hard)

2. Inspect the system to see if this is what it's doing (hard)

Conclusion: too hard, so until I get better I don't know if any given thing is reasoning.

What "very smart people have proposed":

1. Look at something I've only seen people-I-assume-to-be reasoning do

2. Run the system and see if it can do this

3. If satisfied or in the dictionary business, redefine reasoning

4. goto 1