How to differentiate 24 Bit PNG and 32 Bit PNG file?

When image is encoded in PNG format, it can be encoded either in 24 bit format (rgb) or in 32 bit format (argb). Both the files appear as .png files. Then how it is possible to differentiate these two types of png files?

To make it more clear, If I am downloading PNG image files from internet how can check in which format it is encoded.