On Windows Phone, you should read the app description to see if it mentions an in-app purchase or take a look at the reviews by users. Regrettably, mention of In-App purchases on Windows Phone appears to be at the app developer’s discretion so ultimately you may need to install the app to be sure.
How To restrict In-App Purchases
Microsoft have a really great way to protect the kids from purchases via Kid’s Corner feature. This creates a separate access level on your phone to only the content that you want kids to access. Note that by default In-App purchases are disabled for this area entirely, however if you set a Wallet PIN you can approve individual purchases.
Regardless of if you intend to use the Kid’s Corner feature to protect yourself it is a good idea to setup a Wallet PIN on your handset anyway. Go to the Wallet application and tap More > Settings + PIN > Wallet PIN. After setting up your PIN code, check the option that reads ‘Use Wallet PIN to protect music, app and in-app purchases’.