How to check if an app is installed from a web-page on an iPhone?

As far as I know you can not, from a browser, check if an app is installed or not.

But you can try redirecting the phone to the app, and if nothing happens redirect the phone to a specified page, like this:

setTimeout(function () { window.location = "https://itunes.apple.com/appdir"; }, 25);
window.location = "appname://";

If the second line of code gives a result then the first row is never executed.

Hope this helps!

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To further the accepted answer, you sometimes need to add extra code to handle people returning the browser after launching the app- that setTimeout function will run whenever they do. So, I do something like this:

var now = new Date().valueOf();
setTimeout(function () {
    if (new Date().valueOf() - now > 100) return;
    window.location = "https://itunes.apple.com/appdir";
}, 25);
window.location = "appname://";

That way, if there has been a freeze in code execution (i.e., app switching), it won't run.

iOS Safari has a feature that allows you to add a "smart" banner to your webpage that will link either to your app, if it is installed, or to the App Store.

You do this by adding a meta tag to the page. You can even specify a detailed app URL if you want the app to do something special when it loads.

Details are at Apple's Promoting Apps with Smart App Banners page.

The mechanism has the advantages of being easy and presenting a standardized banner. The downside is that you don't have much control over the look or location. Also, all bets are off if the page is viewed in a browser other than Safari.