How to tell if an app used the internet for local file transfer? My friend uses Send Anywhere to transfer files between her Android phone and tablet. The tablet connects to the internet through a hotspot from the phone.

The app is supposed to transfer the files locally, without using the Internet, but now my friend is claiming  that her SA transfers ate up her whole cellular data pack.

How can we confirm this or rule it out?

We have Opera Max, and I think maybe native Android features as well, which tell us how much data each app has used, but the problem is that they can't tell the difference between data being transferred locally to another device on the same WiFi network, and data actually being transferred over the internet. The same thing goes for the Xposed module Network Speed Indicator.

Things I have thought about:


Looking at the transfer speed: Depending of the speed of ones Internet connection, the speed may differ when local connection is used and the internet is used. 
Checking the remaining cellular data allowance from one's operator before and after a transfer.


I have already tried both these approaches on one or two occasions, and what I found is that the transfered happend locally, without the Internet being used for anything other than the innitial "handshake." But the problem is that my friend is now claiming that Send Anywhere comsumed her whole cellular data allowance. If my friend is correct, it would mean that the app is unpredictable and may sometimes use the Internet even though both devices are on the same network (they would have been, my friend has only her one cellular connection and nothing else.)

This is actually not impossible. For example, another app called Airdroid, with some similar functionality, sometimes fails to get a local connection between a hotspot hosting Android device and a connected desktop computer. It then warns me that the two devices are using the internet to connect to each other. However, this only happens sometimes, and refreshing the web app on the desktop device usually solves it.

A similar bug with Send Anywhere may occur, but, like Airdroid, it may only happen occasionally. (It also might not warn me about it, like Airdroid does.) Therefore, it may be difficult to reproduce at any given attempt. Thus, I would like to monitor over an extended period of time how much data each app has used, in a similar way to what Opera Max dues, but I need to be able to distinguish between local data and actual internet data. Opera Max doesn't distinguish. If such monitoring is going on on my friend's device(s), I can be able to see whether it's actually true that Send Anywhere consumed her cellular data allowance.

Any advice?

The devices are both running Android 5.1. The tablet is rooted with Xposed, the phone is not rooted.
This is a common thing. If the cellular data is turned on in the sender device(which creates a hotspot), then it is a wifi which is available to the connecting devices. 

An android phone would consume large amount of data in wifi than in a cellular data. So it can be suggested to turn of cellular data while using the specified app.