This is an aspect of "Eye access cues". It can be reliable, but you have to understand how it works.
In general terms, someone will look upwards when accessing visual information, side to side when accessing auditory information, or down when accessing kinesthetic information. The problem is that those are general terms and not everyone does the same thing. What you want to do is pace them first by asking them to do something and watching the eyes. So if you say, "Think of a happy memory" their eyes will go a certain direction. You can be pretty sure that any time they are accessing genuine memories, that is what they will do. You can then ask them to make up a lie, and watch where their eyes go and pace that as well.
You can always just take the gamble and play the numbers that someone is going to conform to the norm, but you will eventually get caught up with that.