There are some problems with the trial period. It doesn't account for those that don't have specific domain knowledge. From my experience, I have worked with people that have hit the ground running and been really productive straight away, but in the long run have not been as innovative as others.

I prefer to hire a candidate that might take 2-3 months to get productive but have some really innovative ideas, that others in the company might have missed. Puzzle questions do give me a sense of how someone approaches and solves problems, and I think you can infer how they will handle real problems since the building blocks to problem solving are essentially the same regardless of what domain. Solving puzzle questions is just a component of things I look for:

1. Experience in solving challenging problems 2. Good university results 3. Personality fit, easy to get along with 4. Leadership potential 5. Enthusiastic/passionate

Not all five are needed, but candidates need to be strong in more than 1-2 areas.

If you have issues with interviews specifically, you could enter a TopCoder competition, or Google Code Jam, and use that as evidence that you can solve those types of problems.