You will be asked questions as if the interviewer is a customer - basically doing tax research. This is what you will do on the job so it's a reasonable way of interviewing.

I received an email from a recruiter the day after I applied online and after speaking with her, I was emailed a 25 question test that covered many aspects of tax. As a CPA, I found it quite difficult. If you don't have a broad range of experience some of the questions can really trip you up. It is open book, but it is also timed so there is pressure to get through it all.