decrease the input power of receiver as long as the specified bit error rate or EVM is fulfilled. the smallest power value which still fulfills your BER or EVM is your sensitivity.
the sensitivity is the minimum signal input level that your receiver can receive with some characteristic (BER for example). Looking at the method to calculate this level we have:
Si = Ni + (C/N) + NF
Si = signal level at sensitivity
Ni = kTB (for a channel of 8 MHz = -105.16 dBm). You can consider -174 dBm/Hz + 10log(B)
C/N = minimum signal-to-noise ratio to obtain the wanted BER (see the standard)
NF = noise figure