有没有办法检查“是否”；“从未知来源安装”；是否在Android上启用？

如果未启用此选项，我想提示用户。

这里有另一种检查此设置的方法：

Uri settingsUri = Settings.Secure.CONTENT_URI;
String[] projection = new String[]{Settings.System.VALUE};
String selection = Settings.Secure.NAME + " = ? AND " +
        Settings.Secure.VALUE + " = ?";
String[] selectionArgs = {Settings.Secure.INSTALL_NON_MARKET_APPS,
    String.valueOf(1)};
Cursor query = getContentResolver().query(settingsUri, projection,
    selection, selectionArgs, null);
if (query.getCount() == 1) {
    // it's enabled
} else {
    // it's not
}

boolean isNonPlayAppAllowed = Settings.Secure.getInt(getContentResolver(), Settings.Secure.INSTALL_NON_MARKET_APPS) == 1;

此外，向用户显示设置的代码可能会很有用：

if (!isNonPlayAppAllowed) {
    startActivity(new Intent(android.provider.Settings.ACTION_SECURITY_SETTINGS));
}