Fact is, you have to do everything up to that 'mini project' then just go with your gut and hire/no hire. If they underperform in the role you need to either change the role, try and train them up or let them go.
This trial period is just a way of pushing the risk of a bad hiring decision back onto the candidate and allowing the manager to avoid the discomfort of actually firing someone (although telling the candidate that the project didn't work out is functionally the same thing).