  The AWAKE experiment will use a \SI{400}{GeV/c} proton beam with a
longitudinal bunch length of $\sigma_z = 12\,\rm{cm}$ to create and sustain
GV/m plasma wakefields over 10 meters . A 12 cm long bunch can only drive
strong wakefields in a plasma with $n_{pe} = 7 \times
10^{14}\,\rm{electrons/cm}^3$ after the self-modulation instability (SMI)
developed and microbunches formed, spaced at the plasma wavelength. The fields
present during SMI focus and defocus the protons in the transverse plane
\cite{SMI}. We show that by inserting two imaging screens downstream the
plasma, we can measure the maximum defocusing angle of the defocused protons
for plasma densities above $n_{pe} = 5 \times 10^{14}\,\rm{electrons/cm}^{-3}$.
Measuring maximum defocusing angles around 1 mrad indirectly proves that SMI
developed successfully and that GV/m plasma wakefields were created. In this
paper we present numerical studies on how and when the wakefields defocus
protons in plasma, the expected measurement results of the two screen
diagnostics and the physics we can deduce from it.
