Use a razor blade first to get all the large/thick stuff off...Then move onto the Goo-Gone. After that, give your whole windshield a good clean with your traditional window cleaner...And hopefully you know how to properly clean windows. I work in a detail shop, and how we do it is...
1) Use the window cleaner over the whole surface...Spray it on the window since you don't really have to worry about it getting on anything.
2) Wipe it off with the opposite side of the towel (the dry side)
3) Use a Microfiber towel, this will bring the smears and smudges and streaking to and absolute minimum.
To make sure it's clean, take it out into some light, and look at it at an angle, you'll be able to see if anything is left on it.