If you need to check their QA skills then you can ask on following topics :
1 - What is their expertise? Experience in manual testing , automation testing , technical analysis
2 - What is STLC and How he/she follows?
3 - Testing terms
4 - Ask about Test cases , test plan , bug reporting
5 - What is main weakness and strength of that person
6 - Ask like what if he approved software and client found bugs , How he will convince client?
7 - What is BEST and WORST time during their QA job experience?
8 - Why started career as QA?
9 - What is future goal?
10 - What is hobby?
11 - Ask about Bug life cycle
12 - Ask about team management , How they can handle team
There can be lot of , but in my mind above looks important.