The question mark

When editing, I see people misuse question marks quite often.

You should only use a question mark at the end of a question.

The problem is, many think it belongs at the end of a sentence like this: I wonder how many people attended the concert?

That is not a question, and so the question mark should not be there.
You’re stating that you’re wondering the amount, but you are not actually asking what the amount is.

If you were asking, and wanted to use a question mark, it should look like this: How many people attended the concert?
OR Do you know how many people attended the concert?

Question words at the start of sentences can help sometimes. These include: what, when, where, who, whom, why, would, should, could, how, do, etc.
But that can also be misleading since we can ask questions without those words, and we can use those words in a non-question context.

For example, I can ask a question without using a question word: Remember that time we learned about question words in primary school?
Or I can use a common question word without actually asking a question: How we teach grammar is very important.

I know in informal conversation question marks are used a lot more freely (I do it myself sometimes, for emphasis or for comedy), but in formal writing you need to get it right.

Just remember, if you’re not actually asking a question, you shouldn’t use a question mark.