Are you saying that you have to hire candidates without knowing if they can actually do the job?
Which is why some companies are so big on "must have degree" or use a temp to hire service.
Acquaintance of mine wanted to hire someone for his handyman business. He was told that he could not take him on a job to test his skills because that would be looked at as trying to get free work out of him.
You also can not easily hire them just for the day to test them out. The paperwork is killer.
Sane, sensible solutions are squashed by the government, unions and civil rights lawyers.
Back when I was just starting out I went into a business and offered them one free day of labor to show that I did have the skills even if I did not have the piece of paper. Got the job too. Can't do that anymore.