My SDRs include a direct sampling RTL-SDRv3 and a Hermes Lite 2 (among many other SDRs).
I measure my local noise floor (including the combined contributions from QTH RFI, antenna, and receiver noise) by comparing the spectrum magnitudes of the received IQ data from my antenna against received data from a calibrated signal source (an S9 RF frequency generator), and also an uncalibrated (but roughly known power level) noise source as a double check. I then use a set of step attenuators to check my magnitude measurement linearity (set to 6 dB per S unit) down to S0.