A serious client respects your time and efforts and will consider your experience. You will rarely be asked to work for free for the always-insulting promise of “more work later.” If they truly want a test, it will have some rate of pay in exchange for the rights to the ideas rendered.
If you doubt a client, doing some research can shed a lot of information. Look them up on LinkedIn. What is their history? Are there former employees you can send a message and ask about the client’s history with freelancers? Are you a member of an organization where others might know about this client? Post to the group’s discussion board and ask.
Unfortunately, with the state of business and an abundance of freelancers around, clients can ask for tests and plenty will heed the call. They might not be the most professional of the lot, but that’s why many clients ask for tests…they can’t afford the reliable professionals. In the long run, you get what you pay for.