If your images are embedded in Pagemaker, you can't tell. You need to relink them to the original high-resolution images. It's been way too long for me to remember how to do this. This document may help:
Unfortunately, PageMaker doesn't display the effective resolution of the images (not even the actual ppi, for that matter). You'll have to compare the original image resolution and the placed size in PM, then do the math yourself.
For embedded images, you'll have to add an additional step: export the image first. To achieve this, click the image, then perform the File->Export->Graphic command. Choose the location of the new exported file, then verify its resolution. PageMaker will export the graphic in its original dimensions (not the dimensions placed in PM), which is nice in the majority of the cases, but not this one in particular, since you'll be required to do some calculations to determine the effective resolution of the placed image.
As I pointed out in my answer to the OP, PageMaker _has_ an "unlink" feature: it's in the File->Export->Graphic menu. The only caveats are that the menu is disabled when there are more than one image selected (ie: the unembedding process may be performed image by image) and the user has to provide a name and location for every image. But at least the dialog box has a "Link to new image" command, which saves a little time.
@Joao, It's been WAY LONG since I worked in Pagemaker. Thanks for replying to the poster.