you can ask for sample of his work, but that could backfire. Just think for a second, would you want the developer giving away parts of your code to other clients ?
The best you are going to do is this:
- Get references (although these are fungible)
- Type of work the developer has done
- How is the payment ? Agree to this upfront by the hour or by the project.
This is how I do my development for clients:
Agree of estimated cost for the project (with or without source code included) in phases depending on the size of the project.
I develop working software and deliver it with a short expiration or simulation restriction without payment.
Once the client signs off the component/software, then payment is due as agreed and i release the software.