The best response that shows trustworthiness is to tell your manager that you saw a coworker quickly take the cash tip off of a table he was not serving. This is because it is the most honest and direct way to address the situation. It also shows that you are willing to stand up for what is right, even if it means reporting a coworker.
The other options are not as trustworthy. For example, taking $7 out of your tip money to place on the table could be seen as trying to cover up for your coworker's theft. Telling all of your other coworkers that the server who took the money is a cheater and a liar could be seen as gossip or slander. And following the server around for the rest of the night could be seen as harassment.
It is important to remember that trustworthiness is a key quality for any employee. When you are honest and reliable, your employer and coworkers will be more likely to trust you and depend on you.