Tinted Glass Vs. Normal Glass

That was the easy way - no need to even touch the glass! Of course if the film is rather dark, you don't even need any spectacles - it will be obvious just by looking at it. The other way (for light to very light films) is to closely examine the glass viewing it at an angle and feeling the surface towards the edges. If there's a film you will see/feel the film's boundary a few millimeters from the glass's edge.

The easiest way is to sit in the car with windows rolled half down. See through the window.

If the intensity of outer objects (looking through glass) is dimmer than the open window, then the glass is tinted one. You can notice slightest change in a open day light.

That makes life easy .Thanks Rohit and Amrut