The motion of a free liquid surface inside its container is called sloshing and it is strongly affected by external excitations applied to the partially filled container. The understanding and the prediction of this particular motion is very important for space vehicles which are powered by cryogenic propellant. This is the case of a cryogenic upper stage where the dynamic response to sloshing can also interfere on the thermodynamics of the stored fuel. The scope of this paper is to investigate cryogenic sloshing by means of non-intrusive optical techniques and to evaluate their performances. In this work LN2 is used a substitute fluid of a real cryogenic propellant (LH2/LO2). Two techniques will be applied, both needing the use of tracers into the flow. In this paper the techniques are detailed and preliminary results of the liquid/gas interface shape and of the fluid velocity during LN2 sloshing will be presented.