How can you tell whether the file you've downloaded in PNG-8, PNG-24, or PNG-32?

Question

I'm trying to download a picture off the net, and I'm saving it as a png file (image.png). How can I tell whether the image is a PNG-8, PNG-24 or PNG-32 file (is there a default?) and is there anyway of changing the file type to one or the other? I'm using a Windows 10 computer btw.