Is there a way to track if an ALERT email was opened by the ALERT recipient?

Since you are only able to track activity on an individual person (lead), you wouldn't be able to track the actions of an outside person (your rep) for an email alert. The only filter available regarding Alerts is Alert Was Sent and Not Alert Was Sent.

You could always use UTM parameters for specific links in the Alert email but it won't be exact (nor is this really a best practice).

Alert recipients don't even need to exist in Marketo! So the answer, out-of-the-box, is No.

But it's a reasonable need. The way you can achieve this is by sending an actual Email, not Alert. And that's done by posting a webhook to the "loopback" forms endpoint (on localhost) with the Email field set to the sales recipient's email and any custom Textarea field filled with the original recipient's info. Dedicate a form to this purpose because it's clearer ("Alert Triggering Form" or suchlike). Then trigger on Filled Out Form and Send Email in the flow. You can follow the exact process Dan describes here (ignore the title of the thread -- it turns out there was a miscommunication and the method is indeed reliable!).