> You can find out if they copied the homework. There should always be a follow up talk about it. They should be able to explain the details.

That doesn't prove anything. They could've hired a senior developer to help them and explain the concepts in detail.

> Ask them to add some functionality on spot. They have working code they should be familiar with and you can work with that. You want to test their approach more than anything.

The more 'real-world' this is, the more it's going to be biased towards those that have experience in a particular stack or with a particular type of development. Which isn't necessarily bad (it could even be good!), but it may be if you want a more agnostic interviewing process.