How to determine the current locale's symbol for a decimal point

You need to use methods of NumberFormat class, available in intl library.

NumberFormat.decimalPattern formats a number according to specified locale.

e.g. this is how you can get device locale and format a number.

import 'package:intl/intl.dart' show NumberFormat;
import 'package:intl/number_symbols_data.dart' show numberFormatSymbols;
import 'dart:ui' as ui;

String getCurrentLocale() {
  final locale = ui.window.locale;
  final joined = "${locale.languageCode}_${locale.countryCode}";
  if (numberFormatSymbols.keys.contains(joined)) {
    return joined;
  }
  return locale.languageCode;
}

// ...

String dec = NumberFormat.decimalPattern(getCurrentLocale()).format(1234.567);
print(dec); // prints 1,234.567 for `en_US` locale

However, to answer your initial question - here is a function that returns either decimal separator (if found any) or empty string.

String getDecimalSeparator(String locale) {
  return numberFormatSymbols[locale]?.DECIMAL_SEP ?? "";
}

// ...

print(getDecimalSeparator(getCurrentLocale())); // prints `.` for `en_US` locale.

Let me know if this helped.