Stick half a tape cross to a window (Like an F with the top bar missing.) Then stick a round sticker on the place where the two lines join. I suggest using a window because phorias are often triggered by the stimuli of the light. I suggest using only half a cross because the whole cross can be confusing to the brain.
Sit yourself in line with the half cross and relax your right eye. Then look at the spot but note where your eye is really going. Is it staying on the spot, going up to the top of the line, down to the bottom of the line or across the middle bar?
Then do the same with the F backwards for the left eye.
Make yourself a chart and record your data.
Then take a look at the different types of phoria on About Phorias. Well done, You have learnt something about your vision today!