Website shows diamond with question mark character

Was working on a website which was showing a strange character that looks like a diamond with a question mark in. Last time I saw this happen it was corrected by specifying the character set as utf-8 with the following meta tag:

meta equiv="Content-Type" content="text/html; charset=utf-8"

On this occasion the meta tag was already in place and it turns out that the page had a strange single apostrophe. In code view the single apostrophe is usually vertical but this one was slanted. Replacing with a regular one resolved the issue.