Take a picture of the screen while the movie is playing on your computer
Load this picture in an image editing software (like PhotoShop)
With the "rectangular lasso tool", select only the image of the movie
your editing software should show you the width and height of the selection (I know PhotoShop do that)
The aspect ratio is the width divided by the height!
(by the way, it's 1.33:1, 1.78:1, 1.85:1 and 2.35:1, not 1:33:1 1:78:1 etc. because it simply says that if your screen is 1 meter high, the picture will be 1.33, 1.78, 1.85 or 2.35 meters across)