You are looking for an even temperature across the whole width of the surface of the tyre that is in contact with the track. The only practical way of doing this that I know of is to run the car then quickly take readings (if possible before you get back to the pits) across the all driven tyre widths.
If the centre is hotter than the edges then you are running too high a pressure. If the edges are hotter than the centre then the pressure is too low. it is a matter of trial and error.
Differences in temperatures between the tyres on the same axle indicates uneven loading, but that is another and more complicated issue