The way to assess noise level is to feed in a constant 1000Hz tone, set your levels to 0 db, and record 10 seconds of tone, pause the recorder, remove the tone and replace it with a 200 ohm metal film resistor and record 10 seconds of silence.
Then, open the file in a DAW, nomalize the tone to 0db, select a section of the silent part and open it in a spectrum analyser window, ( Audacity has a good one ).
You need to look at the noise levels between 1000 and 10000 Hz, you can do this in audacity by placing the cursor at the required frequency and and reading the noise level on the bottom line.
You can't assess noise levels using the level meter on your DAW, as there is always a lot of very low frequency junk around 10Hz, caused by the internal processor of the recorder, and the level meters will display that, and give a false reading.
Attached is a noise plot of my Tascam DR-40 with the gain set at full.
Audacity can also be used to generate the 1000 Hz tone, but if you are trying to compare two recorders, you will have to make sure that the amplitude of the tone remains exactly the same for both tests.