How to tell if certain pre-2015 app is 64 bit?
The easy way for me is to install the app on iOS 10.3 (or later) device. 

Then open the settings app.


tap general
tap About
tap Applications


iOS will report on the possibility to update all 32 bit apps you have installed on the device by letting you tap to each entry in the App Store where the developer web site is another tap away.