123
234
2456

try {
    AssetManager assetManager = getResources().getAssets();
    InputStream inputStream = assetManager.open("nome-do-arquivo.txt");
    InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
    BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
    String linha;
    LinkedList<String> linhas = new LinkedList<String>();
    while((linha = bufferedReader.readLine())!=null){
        //aqui com o valor da linha vc pode testar o que quiser, por exemplo: linha.equals("123")
        linhas.add(linha);
    }
    inputStream.close();
} catch (IOException e) {
    e.printStackTrace();
}