What kind of substrate you have used ?

metal substrate Bogdanowicz

Ellipsometric measurements will help ;)

What kind of film you have ? oxide, nitride ..?

oxide Bogdanowicz

You could use the colour of the film in some range of thickness. Check -> http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=105&pageid=2144416143

Visual inspection is very accurate method and it is well developed for SiO2 grown on Si-substrate. Color charts can be easily found. However have in mind that the color depends not only on thickness of the film but also on its refractive index and optical properties of the substrate.

There are a number of experimental techniques that can be used: ellipsometry, interferometry, spectroscopy, spectrophotometry and even XRD (X-Ray Diffraction), if you consider a more broad concept of any wavelength. But as R. Bogdanowicz and Tsvetanka Babeva have already noted, a simply inspection by the color of the reflected light you can "estimate the thickness of thin film from the light reflected from it" (i.e., ESTIMATE, don't know with high precision; it depends on the real refractive index of your substrate and your film and, also, the charts are not interpolated for any thickness value, but only for some specific and representative ones -generally of interest for microelectronic process-).

If you're interested on the theory and practice of the colors reflected by a transparent film deposited onto a reflecting surface, you can read:

http://en.wikipedia.org/wiki/Interference_(wave_propagation)

http://en.wikipedia.org/wiki/Thin-film_interference

There are even some excellent images and photos that illustrates this phenomena.

And the color chart for SiO2 films deposited onto Si substrate can be found here:

http://www.htelabs.com/appnotes/sio2_color_chart_thermal_silicon_dioxide.htm

All you need to do in order to get the approximative (ESTIMATED) thickness of YOUR film, is to apply a simple proportion (how many times the refractive index of your film is higher than those of the SiO2).

You're served!