On your iPad you can view your purchase history (the last 90 days), and narrow it down by type (I haven't made any in-app purchases but I assume that they will show as a separate list on it) via http://reportaproblem.apple.com - that site also allows you to optionally contact iTunes Support if you have a problem with a purchase
On your computer's iTunes you can only view a full list of your purchases, you can't filter it by type : log into your account on your computer's iTunes via the Store > View Account menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases