Are hydronic systems set up as loops or ladders, and how can I tell the difference?
The temperature gradient between heaters will be different depending on how the system is arranged.  A larger system is likely a two pipe system with the supply and return piped to each floor.  Get an accurate clamp-on pipe temperature probe and the appropriate meter:



Open all windows, turn all heat full on at all locations and let the system stabilize for 20 minutes or so.  Record the temperature at the inlet pipe and outlet pipe for each radiator.  Highest temperature indicates closest to the supply line.  Coolest temperature indicates closest to the return line.  If the floors are run in parallel between supply and return, then the radiators closest to the supply line on each floor will be about the same inlet temperature.  Follow the temperature gradients and map out your piping.

Edit:  I should mention that this test relies on the boiler running continuously.  If your boiler is cycles frequently because it is oversized, then measuring the system temperature gradients will be tricky.  You will need to pay attention to the varying supply temperature.