The Raman spectrum of bulk WS2 has two prominent peaks: an in-plane (E2g) mode located around 350cm-1 and an out-of-plane (A1g) mode which is located at 420cm-1. The in-plane mode corresponds to the sulphur atoms vibrating in one direction and the tungsten atom in the other, while the out-of-plane mode is a mode of just the sulphur atoms vibrating out-of-plane.
As WS2 becomes single-layer these two modes evolve with thickness. The in-plane Raman mode upshifts, although very slowly (less than 1 cm-1), and the out-of-plane mode downshifts to 417cm-1. This means that monolayer WS2 can be reliably identified by the shift of the A1g mode.
Perhaps a more direct way of monitoring the transition to monolayer, in the case of WS2, is the observation of its photoluminescence. While bulk does not show any photoluminescence, as a result of its indirect band gap nature, monolayer shows strong and sharp photoluminesence which is a signature of a transition to monolayer state.
References
[1] Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2 Zhao et al. Nanoscale, 2013
[2] Identification of individual and few layers of WS2 using Raman Spectroscopy Berkdemir et al. Scientific Reports, 2013
[3] Electrical and optical characterization of atomically thin WS2 Georgiou et al. Dalton Transaction, 2014