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.
Trello – Insert HTML Entity in Card Comment
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Best Answer
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.