A laser thermometer can tell you a lot. You can get them for less than 20 bucks. That will help you understand whether the problem is on the cooling side, or the engine side. With that, you can look at the temperature deltas between the radiator inlet and the radiator outlet. A normal spread is around 10-15 degrees. A narrower delta, like less than 10 degrees, means you don't have sufficient cooling capacity which can be too small or inefficient of a radiator or inadequate airflow through the radiator from a faulty fan or drive assembly. A wider delta -like too cool- can point to restricted flow, meaning coolant is spending too much time in the radiator. You also want to check the whole surface of the radiator. You are looking for wide temperature variations that tell you where there are restrictions.
For the engine side temps, you want to look at what temperature you get at the water pump inlet and the manifold temperature ahead of the thermostat. The delta is very similar. Retarded timing or a lean mixture will give you a wider delta. Low block pressure can do that too as low block pressure can allow steam pockets to form and once you have steam pockets, you aren't cooling that piece of metal. You get low block pressure from a slow or inefficient water pump. Also, don't run straight water of any kind. Water has high surface tension and the electrons don't like to get friendly with metal components so you get poor surface contact. Use antifreeze or a surfactant to break the surface tension.
