The indicator is a non-electric indicator and is based on a thermochromic material which is applied on a diffuser or thermodiffuser device of volatile substances (1), so that when heating of the latter takes place to evaporate the volatile substance contained in a reservoir associated with said device (1), when a determined temperature is reached the indicator, constituted by a plastic insert (4) with thermochromic pigments, changes colour and indicates that the diffuser device (1) is in diffusion conditions. The colour change product can be constituted by the combination of some thermochromic and normal paints applied on the external surface of the device, so that when a determined temperature is reached the thermochromic paint changes colour and originates an indication of the device being in diffusion conditions.