To a certain extent the answers here will be similar to System Administrator interview questions and answers and What are your best senior level Linux interview questions since most any good linux admin would need to know their way around Apache.
However, I'm not hiring a senior linux admin, I'm hiring a firm to secure our web servers, to optimize their performance and decrease both the risk of failure as well as the consequences of same. Being able to walk me through the entire Linux boot process as suggested in one of the responses above seems overkill.
I wonder whether one might choose to ask different questions of an interviewee if he or she is exclusively adminning a few Linux web servers rather than performing deeply complicated configuration of Linux.
Perhaps I'm betraying my ignorance here - please forgive if so!