A smack with a hammer can tell you all sorts of things. If the board is rotten; some is likely to come off. If it's under tension it'll sing out with a resonance absent from a board carrying only its own weight.
In your circumstance, the board should have had some wiggle but prolly wouldn't have.
> I tried to cut it and it started to bind the saw. I stopped and thought, and then did not cut anymore until I had shored up the upstairs.
https://www.jefftk.com/p/accidentally-load-bearing#fb-231219...