Simple you would think, the floor sags push it up again and we are good to go? No stop! You will most likely bring about some more complications. In most cases the building sag has been there for so long that it is very difficult to work out exactly where the problem is.
Looking at the hand sketch following you can see, that as a centre post near a structural wall sinks then additional pressure is transferred to the outside posts. It does this because the roof ties the building in like a sealed box.
These outside posts are subjected to the weather and usually deteriorate more rapidly, however it is inside the house that receives the traffic. Normally a well used hall etc abuts the area of internal structured walls and with heavy traffic it's these posts that will sag first transferring weight to the deteriorated outside posts pushing them into the ground like a sharpened stick or spear.
It can be a case of the chicken and the egg, a post near a staircase on the outside might receive heavy traffic transferring weight elsewhere however it is normally the case first mentioned.
Also a lone stump or group of stumps on the outside that don't sink with the building can put enormous pressure on the centre structural wall and over time timber joints get weaker and timber sags it will look! like is is sinking severally in the centre whereas it's being pushed down hard at that point.
Not withstanding this it needs to be evaluated, it can be done with a trained eye looking at tell tale signs as to what has happened but the simplest is a lazer level and create a model space showing post heights.
This needs to be done irrespective of whether you are completely restumping or peace meal with a few posts at a time. Not necessary if your changing the height of the building as a steel frame can be installed to take all the weight in the order of achieving a lift or a lowering of the building.
If you try to lift it and your wrong you will find extreme pressure on your jacking apparatus. That is if you jack on the outside because you noticed it had dropped in height there but it was the inside that saged first then you will be trying to lift the house taking most of the weight on one point and will most likely result in breaking bearers or stressing timber joints. In this case you would need to work out from the centre to the outside utilising the little bit of give the building has stretched, jointing etc and this will also transfer weight back to the centre as you go.