There could be strain gauges on the pad, but I would imagine it's hard to calibrate them in situ.
More likely, they have lots of sensors on each engine including pressure transducers. They know from single engine tests that pressures in an engine are correlated to the amount of thrust it is producing. A little bit of math later they get the total thrust of the rocket
Reliable_Redundancy t1_j828dmh wrote
Reply to comment by glhope in SpaceX on Twitter: Super Heavy Booster 7 completed a full duration static fire test of 31 Raptor engines, producing 7.9 million lbf of thrust (~3,600 metric tons) – less than half of the booster’s capability by allforspace
There could be strain gauges on the pad, but I would imagine it's hard to calibrate them in situ.
More likely, they have lots of sensors on each engine including pressure transducers. They know from single engine tests that pressures in an engine are correlated to the amount of thrust it is producing. A little bit of math later they get the total thrust of the rocket