The more he probed, the more confident he became that if he was All Blacks head coach in 2012, Foster would be a compatible assistant.
Even the many sceptics would be hard pushed to dispute that Hansen was right in his assessment of Foster.
Selection and development of the backline has been cohesive and progressive under Foster's guidance.
Introducing Aaron Smith to test football was a brave decision. From starting 2012 with Daniel Carter as the only proven test first-five, the All Blacks finished last year with another three genuine choices.
Ben Smith has become one of the world's best utility backs, if not the best, and Julian Savea, unable to catch a cold in 2011, has become the most destructive wing since Jonah Lomu was in his prime.
Strategically the backs have advanced — not always smoothly or relentlessly, but they have finished each of the three seasons Foster has guided them in a better place than they started. Not once have they played dumb rugby. Execution hasn't been faultless, but the intention of how the backs want to play has been clear since 2012.
What's also clear is that an All Blacks backline crammed with veterans such as Carter, Ma'a Nonu and Conrad Smith has built respect and admiration for Foster.
Players of their experience can sniff out a fraud and Foster has engaged them, connected with them and motivated them to keep striving to be better.
These are big qualities and can be used to mount a case that Foster is developing the credentials to succeed Hansen.
But just as easy to argue is that Foster has succeeded at this level because he's suited to being an assistant and not a head coach.
Having specific responsibility for one component does not come with the same pressures or demands of being the overall general. The question would be whether Foster can drive the vision of the All Blacks and then empower his wider management team to deliver it.
Head coach is a different scenario but Hansen is proof that it's easy for those outside the camp to read it all wrong. Having been an All Blacks assistant for eight years, there were few in New Zealand's wider rugby fraternity who felt Hansen was equipped to be a head coach.
Three years on and the All Blacks have lost just twice and Hansen has been world coach of the year in 2012, 2013 and 2014.
Perhaps Foster would be capable of a similar graduation, as it appears knowing the inner workings of the All Black machine is critical.
Knowing the impact of pressure and intensity that test rugby brings are also vital. Succession planning worked with Hansen and it could work again with Foster.
If Foster took over when the time comes, there would be some concern about whether the All Blacks were becoming a closed shop, but that would be preferable to concern as to whether the All Blacks were any good.