One really good place to start is to look at https://steemian.info/witnesses
There you can see if they are running the latest versions, has updated price feeds or if they have missed many blocks. Not everything points to a clear selection process, but it would be a good place to start as well.

It all depends on what you think a good witness would be. Is it one that supports the community with code, hence reading their blog about updates and coding, or one that supports it with support, like contests, promotions etc.

It's all up to you.

But the most important part is that they shouldn't miss blocks, that's bad :)