Harddrive

Hi
I have two questions how do I find out what is on my Harddrive? I am trying to find some information but I would like it in plain English. Also if I take my Mac to someone to help me with some problems he told me he will make a back up of my Harddrive first. How do I know he is not retrieving some sensitive info of mine?
Thanks

Question 1 - what secret stuff on your hard drive do you not already know about

Question 2: He's just being cautious before doing any wok on the drive, and you have just got to ask yourself do you trust the person

I am looking for text message files. Can the person access my banking and other business stuff?

If you're referring to things like credit card numbers and other personal information, yes, if they're on your hard drive in plain text anyone can read them. Which is a very strong reason why you should never store sensitive and personal information in plain readable text on your computer.