Usualy a DBA does not work in "Project" mode, so once they start somewhere they usualy work there for some time (years).

So in the end finding a SQL DBA is not an easy thing and the good known ones are constantly busy.

You have to be a little bit lucky and have some kind of test to make sure they are really what they say (your test is a very good idea), being a sql DBA is very broad, even developer who worked with SQL Server could say he's a DBA just because he think he know some SQL statement.

On top of your test i would add the good old Fizz Buzz, ask them to tune a single query on 2-3 tables (with an index with an include or two missing), backup and restore a DB, create 100 Logins (to see if he can automate things), nothing that couldn't be done in less than 5 mins and look how they do it.

If the guy end up with a rbar solution for FizzBuzz it wouldn't be too bad but one that ends up with a set based solution will have a nice bonus on the final review.

Anyone doing a backup/restore with SSMS wizzard will have a (very) bad point (unless he uses it just to generate the script, then he's lazy -> good point).

Anyone opening the books online will have a good point.

Anyone creating more than 2 Logins without a script will have a bad point.

Anyone tuning the query by not looking at IO and Time statistics and query plan would immediately fail.

With that kind of test it's kinda easy to see if someone is used to work with SQL Server full time or if it's just a hobby.