Since this is 70% helpdesk, ask some situation type questions to see how they think. The technical answer is not important, more the process.
Q: "A local user within the office cannot open a program, its a program that you have not used before, they have a request to the helpdesk to fix it and get it working, how would you find the answer and solve their issue?"
A: "Ask my manager." - not good
A: "Erm, maybe, erm... well, erm..." - not good. (Depending on what you are after if entirely new to IT)
A: "I would visit the user and ask them a few key questions. 1) has the software previously worked, or is new 2) if they can show me how to replicate the error 3) If other people use the software and if they are currently using it totally fine. That shows me the issue is located with just them, or is a larger group issue, and confirms if the software previously worked or never worked for them. If the whole team cannot access the software I would check our knowledgebase to see if a central server controls the software to make sure its up. If its only them, I would check they are connected to the network bla bla bla..."
Seeing how they think helps. The tech stuff can be taught.