The term doesn't really make sense with regard to rasters. Well, some files like TIFFs have a PPI, but its really just a hint if you print it, but that's more format specific. So if that's what you want, you'd only be able to get that from the original file, and that will only be possible if it's a linked image.
If you want to know what PPI the document is using for rasters, I'm not actually sure how one gets that information. You can set it when you create a new document, but after I don't think you can get that information (just flat out missing from the API unfortunately).
