Since no one in your pool of candidates have experience in Symbian, the ability to pick up a new platform quickly and start producing will be crucial.
Look at their past experience. What operating systems have they worked with? One? A few? Any mobile OS? How did they act when they were confronted with a new platform? Normally if a developer has experience from several operating systems, he would be able to recognize patterns and concepts - many of them borrow from each other.
This goes for programming languages as well. Since you say "application" I'm going to assume that it will be mainly written in Qt, which borrows a few things from Java. Developing in Qt is a lot quicker than developing in native Symbian C++.
If you really need to have some code in native Symbian C++ I would suggest that you check that they:
- Understand memory handling and the concept of the cleanup stack
- Understand the leave mechanism
- Understand Active Objects
- Understand descriptors
These are some of the fundamentals of Symbian but if you include them all in your test application it will not be very simple.