Hello Saja
Abhishek gave you some good links,i just want to add that if you want measure the crostalk in the WDM systems then for FWM you can transmit high power using two channels(only for example) from fiber and attach spectrum analyzer at just after MUX and after SMF.You will notice the 2 another peaks appeared ,that is your FWM and you can measure power of side peaks from scale or by selecting the markers.I have given link from optisystem example explaining this.
For SPM you can also go to the link given below and explaining the concept of SPM as
A pulse propagating through a nonlinear medium undergoes a self-frequency shift due to self-phase modulation. The front of the pulse is shifted to lower frequencies, the back to higher frequencies. In the centre of the pulse the frequency shift is approximately linear.