There's a site specifically for Code review. However, if my question is very Drupal specific, is asking questions like the following appropriate on Drupal Answers?
I wrote function/module X, please review it for Drupal best practices
If you are asking for a full module, I would say that is not fine. It would be better you focus on specific parts, especially to avoid the question becomes a wall of code that would not probably be helpful to many users.
Differently, a single function would be more acceptable, especially when it is a hook implementation. A single function is probably more re-usable, and what reported could be useful to more users.
Generally speaking, a question like, "please review it for Drupal best practices" is not even a question, but a request. The OP should make clear why the code doesn't seem right, or for which reason the OP thinks the code could be optimized.
A 'first' level of code review might be to use the code review module. It will review your module or really anything in the scope of where it's installed. Basically it will alert you to anything not up to drupal standards.
I do this followed by opening up my bitbucket or github account repos to a few trusted associates.