There is a global boolean kIsWeb
which can tell you whether or not the app was compiled to run on the web.
Documentation: https://api.flutter.dev/flutter/foundation/kIsWeb-constant.html
import 'package:flutter/foundation.dart' show kIsWeb;
if (kIsWeb) {
// running on the web!
} else {
// NOT running on the web! You can check for additional platforms here.
}