Thank you for your quick reply @bkhanaliloo.
As a starting point, I have simulated SPP excitation on the interface between a semi-infinite gold film and air. In this simulation, I used a electric dipole source to excite SPP. As you said, I the field components (Ez or Hy) can be measured by DFT monitors; however, the field of SPP is disturbed by the field radiated from the dipole source (i.e. concentric field is superposed to the SPP field). This makes it difficult to measure the spatial extent of the SPP field (by a line DFT monitor placed normal to the gold surface).
Additionally, the propagation length looks much shorter than analytically obtained one.
This graph shows the analytically obtained |Hy| of SPP (at 1nm from gold surface) as a function of propagation distance.
This result has been calculated based on below equation about TM field and the dispersion of SPP (from Plasmonics: Fundamentals and Applications (S.A. Maier) https://www.springer.com/us/book/9780387331508).
This graph shows |Hy| simulated using FDTD solutions. The dipole source is placed at x=0 nm.
Obviously, the attenuation of SPP is much faster in the result of FDTD solutions. I wonder this difference comes from the method of excitation.