Here is a laundry list that you can look at:
1. Commitment - Has the applicant been jumping from one job to another?
Unless they are freelancers and only takes short term projects, you can ask how many companies they were with when they were still in a corporate industry. With most VAs who used to work in a call center before, they might be jumping from one center to another which might be an indication that they might have a commitment issue.
2. Communication - You can ask anything here. (eg: Tell me some about yourself; What are strengths? Weakness?)
You can validate how they speak English as the language. You can throw check how structured they are in terms of answering your questions. You can also check how fluent they are -- some can speak like a native American since like I said, they used to work in a call center before. This is important if they will be working on descriptions and the likes.
3. Practical Application - Give them a task or two and see their resutls
You can verify how creative they are and how they think out of the box. The training may look easy but at some point, you may go through situations that are quite complex. You would like to look for a VA who is strong thinkers and problem solvers.
There is no really black and white process of how to hire an ideal VA but above mentioned can be helpful pointers for you.