IPhone : How to detect software keyboard was displayed on another apps on iOS?

I'd like to know how to detect software keyboard was displayed on another apps on iOS. Situation is below:

1.Showing alert dialog with OK button on our app.
2.If user taps OK button, 'App Store' and our app's page will be displayed. 
3.User shows software keyboard on App Store. 
4.User returns to our app.

I'd like to detect whether user showed software keyboard on App Store or not on sequence no 4.

Our project manager wants users to write app store's review and wants to know whether user wrote review or not.

But it's impossible to detect another app's events. So alternatively, I want to know whether user showed software keyboard on App Store or not.

If user showed software keyboard on App Store, we can judge that 'user wrote review', it's enough.

Some apps implement this function so I think there are some tips to realize it, but I wasn't able to find good informations or articles.

Does anyone know how to realize it?