The command for trim can return info which tells you the coordinates of where the image was cropped and the new dimensions of the trimmed image. I couldn't find an example for how to do that with this class, so here's how I did it:
$image = new Imagick('image.png');
$imagePage = $image->getImagePage();
list($x, $y) = array($imagePage['x'], $imagePage['y']);
$image->setImagePage(0, 0, 0, 0);
list($width, $height) = array($image->width, $image->height);
This will leave you with two variables for the width and height of the trimmed image, plus two variables for the x-coordinate and y-coordinate of the trimmed area relative to the original image.
This information is useful when the image you're trimming is the difference between two images (e.g. from Imagick::compareImages(), and you want to crop the original image to the same size and position as the difference.