Check if app uses in-app purchase What is the easiest way to check whether an app uses in-app purchase? Is there a mark in the Play store or a permission to look for after I installed the app?
There is a special permission for Google Billing (technical name: com.android.vending.BILLING), which is required if the app wants to use Google's billing system. So if this permission is requested by an app, it most likely uses some in-app-payment (why else should it request this permission).

However: the absence of this permission does not necessarily mean there is no in-app-payment used with an app, as there are other means of accomplishing the like (e.g. premium SMS, or calls to some premium numbers). But if an app requests neither to send SMS, make calls, access the Internet, nor Google Billing, I know of no way it could accomplish in-app-billing without the user really actively doing something, like buying a gift-card and entering the code.