How do I tell if a layer is solid

Duplicate the target layer. Filter > Blur > Average. Change to Difference blend mode. Check the histogram for the mean or median value that is not equal to zero. Remove the dupe layer.

/*
Check If Active Layer Is a Flat Color.jsx
v1.0, 22nd May 2023 - Stephen Marsh
https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-do-i-tell-if-a-layer-is-solid/td-p/13806717
Special thanks to SuperMerlin for the Histogram Mean value code:
https://community.adobe.com/t5/photoshop-ecosystem-discussions/smart-exposure-compensation-script/m-p/11092968
*/

#target photoshop

if (app.documents.length > 0) {

    // Selected layer check by jazz-y
    s2t = stringIDToTypeID;
    (r = new ActionReference()).putProperty(s2t('property'), p = s2t('targetLayers'));
    r.putEnumerated(s2t("document"), s2t("ordinal"), s2t("targetEnum"));
    if (executeActionGet(r).getList(p).count) {

        function main() {

            dupeLayer("_Temp_Diff-Check");
            activeDocument.activeLayer.blendMode = BlendMode.DIFFERENCE;
            Avrg();
            var mean = meanHist(activeDocument.histogram).toFixed(0);
            if (mean != 0) {
                var notFlat = "The layer isn't a flat color!";
                alert(notFlat);
            } else {
                var isFlat = "The layer is a flat color!";
                alert(isFlat);
            }
            if (activeDocument.activeLayer.name = "_Temp_Diff-Check") {
                activeDocument.activeLayer.remove();
            }
        }

        app.activeDocument.suspendHistory("Flat color check", "main()");

    } else {
        alert('A layer must be selected!');
    }

} else {
    alert('You must have a document open!');
}


function meanHist(hist) {
    var acc = 0;
    var cnt = 0;
    for (var i = 0; i < hist.length; i++) {
        acc += i * hist[i];
        cnt += hist[i];
    }
    return acc / cnt;
}

function dupeLayer(layName) {
    var s2t = function (s) {
        return app.stringIDToTypeID(s);
    };

    var descriptor = new ActionDescriptor();
    var list = new ActionList();
    var reference = new ActionReference();

    reference.putEnumerated(s2t("layer"), s2t("ordinal"), s2t("targetEnum"));
    descriptor.putReference(s2t("null"), reference);
    descriptor.putString(s2t("name"), layName);
    descriptor.putInteger(s2t("version"), 5);
    list.putInteger(6);
    descriptor.putList(s2t("ID"), list);
    executeAction(s2t("duplicate"), descriptor, DialogModes.NO);
}

function Avrg() {
    var c2t = function (s) {
        return app.charIDToTypeID(s);
    };
    executeAction(c2t("Avrg"), undefined, DialogModes.NO);
}