Some amazing well thought answers here guys (ya gotta get a life!
) But in real simpleton (me) terms chock the engine/machine stick a vertical weight measuring device behind it much like a large set of bathroom scales & blast away....what the screen reads is what the thrust is, provided there's anything left of the screen:-) Or you could just tie the engine/plane to a monster set of fish scales & let the beast pull it's heart out:-) All other complex factors as mentioned here are worked well b4 us drivers push up the thrust levers or is that power levers?
As Mr Newton once said .............for every action there is an equal & opposite reaction.........Eg you hit me in the face & I'll hit you right back
You stand on a set of scales & it says 100kg (fat bugga) so yr producing a 100kg's of thrust:-)
Like as has been mentioned the book figures are at S/L under certain conditions, what you get on the day when yr push those levers fwd is anyone's guess just like the T/Off weight of a plane at any given time, no one knows exactly what it is!:-)