I think you are referring to a handheld prism based spectroscope with an eye-view/eyepiece. Using such spectroscope one does not observe Raman signal but only observe the dispersed light coming from an object giving information about the absorption characteristics of the object or emission characteristics (for light sources).
The following is such device (personal photo).
Here is a ray diagram for understanding the working principle. White light is the light entering the device. Yellow lines are normal. (Other ways to achieve this are possible, hence this ray diagram is only an example.) On the right, is the image taken from the eyepiece of the device using a cell phone. Sun light has continuous spectra, while white fluorescent lamp (Hg vapor based) shows distinct lines. (Ray diagram is drawn by me. Images shown on the right are captured by me.)
Such device works using prism(s) to achieve dispersion. The amount of dispersion provided by the spectroscope is a parameter governed by the prism used. Higher the dispersion, more fine grained are wavelengths of light dispersed and in such cases one may need to move the eye or (prism) to see full range of spectrum.
Prism based spectroscopes are frequently used for studying gems and diamonds. White light which passes through such samples undergoes absorption (usually by the minor impurities, which imparts color) and the dispersed light gives this information.
You can easily make such portable spectroscope using combination of small optical quality glass prisms. (Or you can just buy, available on Amazon and some companies dealing with optics sell these.)
About observing Raman signal using eyes, it is very difficult since Raman signal is very weak, but it is possible. As per my knowledge no commercial product exists which can do that.