Pay attention to the slight movements of your eyes as you think about your favourite childhood cartoon character. Or a song you’re sick of hearing. Imagine how you may look in 20 years time. Your favourite ice cream flavour? It’s likely your eyes shifted slightly as you thought because your eye movements are linked to certain areas of your brain. Someone trying to lie about a made up conversation may shift their eyes to the left or someone having an internal debate may have their eyes cast downwards to the right.
Of course, this is not completely foolproof as our eyes are constantly moving and are affected by a number of environmental stimuli such as light, sound or even pressure to maintain eye contact in a conversation but this serves as a general guide.