I am opening this post for clients who are asking for a free trial works, So humbly request to my fellow freelancers to raise a flag for asking for a free work since it is against upwork policy.
Have a safe upworking
My suggestion to clients who need to verify a potential freelancer's skills is to set up a small paid milestone with similar work the job requires. Nobody should have to work for free. Yes, this may mean you spend some money on freelancers you do not end up choosing, but it is in line with Upwork's policies and good business in general.