How do I handle a "test project" that feels like free labor?

[Btaliat]

There are sometimes that clients may want to take advantage of test projects on the freelancers. That's why it is better for the freelancers to make sure that you, as a freelancer, is only working towards being firm for you not to be taken advantage of. You should be looking for red flag to know if the clients are actually using you to create the original contents hiding under test job. What do you think about this?