How do you differentiate between rms and peak voltage?
Noting this: -



When your circuit says this (my words in red): -



In the absence of any other information, AC voltages are always presumed to be RMS. 

See this Wiki reference: -


  the magnitudes of the voltage and current phasors V and I are the RMS
  values of the voltage and current, respectively). 


And...


  I managed to calculate a value


Well, unless you have decided to convert to sinusoidal peak values, you will have calculated a value based on an input of 8 volts RMS.