First, make sure that your question is on-topic for this site. In short, questions seeking open-ended feedback on real working code that you own or maintain are on-topic for Code Review.
Here are some examples of questions that are unsuitable for Code Review.
Troubleshooting, debugging, or understanding code snippets
Code Review is for open-ended questions about code that already works correctly (to the best of your knowledge). Questions seeking help about debugging or understanding code are off-topic for Code Review, and may be on-topic for Stack Overflow.
Best practices in general
It's OK to ask "Does this code follow common best practices?", but not "What is the best practice regarding X?" Questions should not contain purely generic, hypothetical code, such as
if condition_a: do_something() else if condition_b: do_something_else() else: do_something_even_different()
Also watch out for hypothetical wording, such as "Imagine" or "Let's say we have…"
Higher-level architecture and design of software systems
We review code, not concepts, diagrams, or outlines. Whiteboard-style questions may be suitable on Software Engineering if they are specific.
Tools and tips for conducting code reviews
Questions about code review methodology should be asked on Software Engineering.
For questions about the Code Review site itself, please visit our meta-discussion site, where you can talk about things like what questions are appropriate and what tags should be used, suggest features, point out bugs, or generally discuss how this site works.