See your purchase history in the App Store or iTunes Store [iOS, computer, or web] - Does not include purchases made by others under Family Sharing; those have to be checked from their accounts.
http://support.apple.com/HT204088
If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - "Before you contact Apple to request a refund for a charge you don't recognize…"
https://support.apple.com/HT201382
If you think there really is something that shouldn't be there:
Report a Problem - "Sign In for help with iTunes purchases made in the last 90 days"
http://reportaproblem.apple.com
Other ways to contact iTunes Support:
https://www.apple.com/emea/support/itunes/contact.html
or
https://support.apple.com/country-selector/itunes