I like to ask, why you want to gauge the knowledge? Is it for salary increase, finding out their learning potential, deciding who is going to be team leader etc? I would just let them work in the team in any case.

If you want them to become more useful to you, ask them to write tests for each others code. That way you can have better longer lasting code. You will have team that is greatly work in cooperating each other and appreciation for others. You might want to regularly introduce new books and provide better environment which are relevant to their work.

The proper way to gauge the skills is to ask them to get more skills. It is beneficial to both.