I got this idea from reading the Vol 14 Side Story. Basically, in c211p12, when Suteguma starting talking about how the average person weighs 60kg and has 4800ml of blood, it's possible that he was saying all these numbers in order to incite some sort of reaction from Baku, and then observe the reaction to see if the number Suteguma calls out is what Baku has.
He said things like: 60kg, 4800ml, 13%, 11, 1 in 9.
The numbers can be separated into: 1, 3, 4, 6, 8, 9, 10 (the 10 comes from how 13 and 11 are pronounced in Japanese).
These are the missing numbers: 2, 5, 7
Suteguma says in this chapter that, because of how the guess '10' failed, he now knows Baku's number. This means at that point, he must've already narrowed things down to 2 numbers, and '10' failing narrowed it down to the final number. So perhaps Baku did not react to any of the numbers that were said at this point. However, there are still 3 missing numbers, not 2 - for that, my guess is that it's possible that he could've crossed out '2' or '5' by using japanese words. For example the sound 'go' for '5' appears in the word 'number' (bangou in japanese)' The sound 'ni' for '2' also appears often in Japanese as a particle, it's possible it was used here.
So my guess is that
a) Suteguma used sounds to incite a reaction from Baku, and used it to cross out the number of beads
b) Suteguma's number must be the inversion of 10 using the missing numbers, so either 8, 5, or 3.
The question that remains is how exactly he measured or observed this reaction. My guesses would be some sort of extreme hearing (such as hearing heartbeats) or eyesight (perhaps being able to observe very small movements and observe nervousness from that?). I'm still not quite sure how this all fits with the Poker game from before, though.