There currently is no way to view the list of options for in-app purchases in the Google Play store itself. However, Google Play does actually know what in-app purchases are available to any given application because all in-app purchases have to be registered in the Google Play Publishers Console before the In App Billing (IAB) platform will function for those items.
Each in-app product has to be registered in each application for which it applies. The process to set up a single product is quite involved. Google Play does have the information available but they don't, at this time, make the list of items available for purchase to users of the Google Play store. That is up to each application. In addition, making that information generally available to users might lead to upset users. For example, a developer decides to add a new highly-requested feature that they initially decide will be $1.99 but then decide later on to raise that price to $2.99 just before releasing it. Those expecting to get it at $1.99 because they saw it early on might be upset at that change. Also, if a developer utilizes the IAB Products API to synchronize a product list from another tool, it might push products into Google Play that are not actually used in the app itself.