How does one fix an unlevel, poorly-supported floor when you can't access it from underneath?
Before I suggest how to investigate your problem, my guess is that one (or more) of the concrete blocks with a post/pier that is supporting a joist or beam has settled. The solution to this can range from removing the flooring in a small section and resetting the block, to realizing that all of the blocks are out of whack and maybe the ground the building is set on was not properly prepared for supporting a building.  You can find a couple of great guides to building large 'accessory buildings' (sheds, studios, etc) on Fine Homebuilding, Family Handyman, etc, but the process generally starts with making sure the ground underneath has proper structure and adequate drainage (e.g. gravel/baserock tamped down, filter fabric, etc).  Depending on the age of the building this might also be normal (I wouldn't expect a building sitting on small concrete blocks (instead of a solid concrete foundation) to last 20-30 years and have a flat floor).

For tackling your specific question, the general idea would be to try to remove a small section of the parquet floor and subfloor where there is a problem (soft spots or out of level floor) without cutting through the joists or beams, in order to better assess the situation.  Once you have that section removed, you should be able to see the exact construction and state of the supporting members (joists/beams/concrete blocks) which should help to identify the cause of the problem.

To identify where the beams/joists are so that you don't cut into them, you could use a mirror and flashlight on the outside of the building, and take approximate measurements.

Another approach would be to just cut right into the soft / out of level area, as you will likely need to repair replace that anyway.

If you removed the parquet floor first, it should be really easy to see where the joists are by looking at the nailing pattern in the subfloor.  Subfloors are nailed to the joists (typically a builder will put the subfloor down on top of the joist, snap a chalk line on the subfloor that is at the center of joists, and then nail along that chalk line).  So you could cut a section of the subfloor out in between the joists and check out the problem.

Since you asked how you would work on a floor from above, all of these suggestions about cutting into the floor would need to be done from above the floor, i.e. using a circular saw, hole saw and reciprocating saw, oscillating saw, etc.  Once the floor is off and you see what the problem is, you would fix it and build the floor back up.  For example, maybe you would decide to just add shims under the joists, or maybe you would put in a new concrete block and post and a new joist or blocking, and then install new subfloor, then finish flooring.  I hope that helps, please let me know if anything is unclear.