I've only done this a couple times (with ?mkt_tok style links), but you could try opening a new browser window and either clear your cookies first or use the browser's private browsing feature (makes a cookie sandbox) so you don't confuse marketo and tie that person's lead to you by accident. Anyways, in your cookie-free browser, click the link and then go to a form that Marketo prefills for you and voila you can see the email address that was originally targeted. There are some ideas floating around out there to let us look up leads by various id's in the SOAP API.