Assuming you're talking about steganography and not hiding the text in the image attributes, then you simply can't.
The whole principle of steganography is to replace parts of the image with your message. Once you've overwritten these parts of the original image, there's no way for you to know what it was. The original information is gone.
Now, if you implemented your algorithm properly, the change is not perceptually visible, but of course if you ask a computer to compare the exact intensities between the two images, it will see the differences.
