Shortcut to find shortened text boxes

Not doable via Grep but script may help. Try the following code.

var doc = app.documents[0];
var tolerance = 0.5; // in points — tweak if needed

for (var i = 0; i < doc.pages.length; i++) {
    var page = doc.pages[i];

    var mp = page.marginPreferences;
    var top = page.bounds[0] + mp.top;
    var bottom = page.bounds[2] - mp.bottom;
    var left = page.bounds[1] + mp.left;
    var right = page.bounds[3] - mp.right;

    var marginWidth = right - left;
    var marginHeight = bottom - top;

    if (page.textFrames.length > 0) {
        var tf = page.textFrames[0];
        var gb = tf.geometricBounds;
        var tfHeight = gb[2] - gb[0];
        var tfWidth = gb[3] - gb[1];

        var widthDiff = Math.abs(tfWidth - marginWidth);
        var heightDiff = Math.abs(tfHeight - marginHeight);

        if (widthDiff > tolerance || heightDiff > tolerance) {
            alert("Page " + page.name + " has a mismatched text frame.");
        }
    }
}

Do note the following

1. The script needs an open document in InDesign
2. The tolerance for variation in size is .5 pt
3. The script assumes that we have only a single textframe on each page
4. The output is in the form of an alert on which pages have the problem.

-Manan