engine code question
 

Is it true that the computer stores codes if if the engine light never comes on?
If that's true, then how do you know you have a problem if it's not going to tell you everytime?

For instance: my '88GT when you start the engine when it's warmed up, it will stall if you don't give it some gas quick or it will idle rough until you hit the gas.
My engine light has never come on but I was told it probably stored a code anyway.
Is there any parts stores that will scan codes for you or let me borrow one?