yeah I delete all of my emails :O
I've taken a day and just gone through ALL of my purchases one by one. You know, I've even found one or two along the way that I thought I downloaded but I don't have on my computer! You never know what you might find going through your history, one by one.....but yeah, that's how I do it. The hard way
Another thing you can do is find out when it released and check your coordinating purchases around the time of release. I've found purchases this way too