When you work with images in your layout, you need to be aware of the effective resolution of those images. The effective resolution is the resolution of the image after it has been scaled. You also need to know if you've unintentionally stretched the image non-proportionally. InDesign has five ways to tell you if an image has been scaled, by how much, and if the scaling was proportional. Choose the method that is most convenient for you.
1. The Control panel or Transform panel (CS3/CS4)
The Scale fields in the Control and Transform panels will display how much a selected image has been scaled, but only if the image is selected with the Direct Selection (white arrow) tool. In the screen shot below, the selected image has been scaled non-proportionally: 76% horizontally and 80% vertically.
2. The Info panel (CS3/CS4)
The Info panel (Window > Info) will display both the original resolution and the effective resolution for a selected image. In the example below, the selected image has been shrunk non-proportionally, so the effective resolution is different in the X and Y directions.
3. The Links panel (CS4)
The Links panel displays not only the original and effective resolution of the image, but also the scale percentage. If these values aren't displayed in your Links panel, you need to choose Panel Options from the Links panel menu, and place a check mark next to these items in the Show in Link Info column.
4. The Preflight panel (CS4)
The Preflight panel (Window > Output > Preflight) can alert you when images have been scaled non-proportionally, or when images don't meet minimum effective resolution standards. To use this, you will need to choose Define Profiles from the Preflight panel menu, and select options for Image Resolution and Non-Proportional Scaling of Placed Object in the IMAGES and OBJECTS category.
5. Preflight & Package (CS3/CS4)
When you choose File > Preflight in InDesign CS3, or File > Package in InDesign CS4 and click on the Links and Images category on the left side of the dialog box, each image in the file will be displayed on the right. When you click on an image in the list, the resolution and the effective resolution for that image will be displayed.