In addition to the others, and probably more specifically your question on ‘how do you know the speed’ - it totally depends on the bike, rider and conditions, and you dont actually know for certain.
Basically you have to try and see - it is ‘blipping’ the throttle though. All on the principle that for a given revs, say 8,000, the bike will be doing a different road speed depending on the gear, e.g. in 4th 8,000 might be 80mph, in 3rd it is 70, in 2nd it is 55, and in 1st 45 . (OK OK this is probably an NC30 here, but stick with it), so as one brakes, the revs needs to slightly constant (or rising) in order to ensure that the engine speed (revs) matches the speed of the bike (mph).
To test this out Ride on a straight road with not much about, get to 3rd gear and say 8,000 revs (assuming a 12,000 redline, change to suit yours), now start breaking, not massively hard, and the revs should start coming down.
Now if you were changing down a gear before you came to a standstill, you need to increase the revs during the gearchange to ensure that the biek doesnt lurch. To do this, pull in the clutch, give the throttle a quick blast (literally 1/4 or 1/8 turn open and shut) while you change down, then release the clutch. Now you probably wont get this straight away first time (bike might lurch), but will give you the principle.
Practice whiel you are coming to traffic lights, etc then you will soon get the hang of it. The principle is that you can go into a corner faster (higher gear) brake, keep the revs high and bike steady as you select a lower gear, progress through the apex and have good drive for the exit of the corner (lower gear)
HUmm, must be bored at work…