How to determine if the user has modified the default font size in flutter mobile app?

You can determine if the user has modified the default font size in the flutter mobile app using MediaQuery textScaleFactor. 

import 'package:flutter/material.dart';

void main(List<String> args) {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});
  @
override

  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Scaffold(
        body: MyWidget(),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  const MyWidget({
    super.key,
  });
  @
override

  Widget build(BuildContext context) {
    var fontScaling = MediaQuery.of(context).textScaleFactor;
    return Center(
      child: Text(
        '$fontScaling
',
        style: const TextStyle(fontSize: 50.0),
      ),
    );
  }
}

Hope this is useful to you.