This is a bit of a noob question. But I wanted to confirm the opposite situation:

Booked on 016 stock - ORD-SFO on UA and then SFO-NRT on NH,

UA checked me in and issued BPs for both flights at T-24 for the ORD departure. We have a fairly tight connection at SFO and the last thing I want is to arrive at the gate to heard "sorry, Mr. Milepig, you don't show as checked in." Is there a place on the NH website where I can see check in status. I'm not immediately finding it, but that may be since the SFO departure hasn't yet hit the T-24 time.

Update

I just answered by own question. When the NH flight time passed I was able to go through check in on the ANA site and was led to an "already checked in" page along with options to print or email a copy or to send it to my Apple Wallet.

So, check in via UA worked.