According to the markdown syntax guide you can just insert HTML entities directly (e.g
© for ©). However, HTML entities don't seem to work in Trello comments (which use markdown)—the entity code appears as-is.
Instead of using the HTML entity, you can insert the character itself in Trello list titles, card titles and descriptions (and even comments, which do not support Markdown). Usually, when I need a special character, I do a web search to find the a website that uses the sought character, copy it (CTRL/CMD+C) and paste it where I need it (CTRL/CMD+V). That's what I just did to insert the tick character here: ✔! (@Spam n Eggs, clearly the same can be done for Stack Exchange comments).
There are plenty of websites that provide charts of special characters, like this one.
If you'd like a handier way of inserting the tick symbol, you could:
Try an autocorrect/text-replacement program. These will run in the background and work across programs. Many find them highly useful for many typing tasks. Create a rule so that whenever you type a certain string (i.e "[tick]") it's replaced with "✔".
If you're on a Mac, you can access the Character Viewer directly from the menu bar to insert characters. On Windows, it's the Character Map.