Separation Preview Setting

If you have View>Overprint Preview turned on - then it will be visible.

I do the same as @Bill Silbert and put the panel active in my layout of panels - all the time. 

But this option is grayed out with no documents open

I came up with this script that will set the Overprint on for New Documents and if you open a document

if you put in a 'startup scripts' folder it will run all the time

http://kasyan.ho.ua/tips/indesign_script/all/start_up_folder.html

On my computer it looks like this

Save the below in a plain text file and name it

auto_overprint_on.jsx

(function () {
    // Function to turn on Overprint Preview
    function turnOnOverprintPreview(event) {
        try {
            var doc = event.target;
            if (doc instanceof Document) {
                doc.layoutWindows[0].overprintPreview = true;
                $.writeln("Overprint Preview turned on for: " + doc.name);
            }
        } catch (e) {
            $.writeln("Error: " + e);
        }
    }

    // Add event listeners for opening and creating documents
    function addEventListeners() {
        $.writeln("Adding event listeners...");
        app.eventListeners.add("afterOpen", turnOnOverprintPreview);
        app.eventListeners.add("afterNew", turnOnOverprintPreview);

        // Apply Overprint Preview to already opened documents
        for (var i = 0; i < app.documents.length; i++) {
            turnOnOverprintPreview({ target: app.documents[i] });
        }
        $.writeln("Event listeners added.");
    }

    // Persist the script state to maintain event listeners across sessions
    #targetengine "session"

    $.writeln("Starting the script...");
    addEventListeners();
})();