Here's what I have been thinking, get a system up like one of those livechat boxes that popup but actually staff it yourself, only have it popup after they click on the offer on the lp. Once they start going through the process you can ask them what it says on the last page (Talk with them as well if they have any questions) after they sign up and enter all the info, that should effectivly could as a lead everytime, if its not showing up then you know something is wrong. Don't get me wrong, this would require you to probably talk to these people but it should give you a really good idea on what % of your leads are actually getting counted.