There is no way on the Android client side to check the diff unless you store the previous version and do the diff manually on your side. However,
Firebase remote config does provide diff option now. Go to Firebase remote config and do the following:

Click on the history icon next to the Add Parameter button it will show something like the following:

Select an item in the history to from which you want to see the diff and then on the other item, hover your mouse curser and location the 3 dotes menu and select compare with selected version.
Firebase will show you the diff and you can choose for show differences only option to see the changed item alone.

Note: I could not find the whole value diff even after coming to this stage of diff. If you config value json is long and then you can not really see the whole difference. It would have been better if Firebase had provided complete diff option.