How to find areas where heat/coolness are lost?
Whether for heating or cooling, your goal is to limit the amount of heat going through the building envelope, it's just the direction the heat is moving is reversed. Cooling loads are complicated by radiant heat gain which is not much of an issue for heating loads. Thermal imaging can identify areas of high heat conductance, indicating where more insulation is required.

Other than that, simple inspection can indicate where the heat is getting in (or out). Are there any gaps in windows and doors allowing air to leak in or out? If the windows are singled glazed, that will be the largest single cause of heat gain. How much insulation is in walls and ceilings? What's good for heating is good for cooling. In addition for cooling, how is the sun allowed to radiate heat into the building. Lack of shading devices, window tints, draperies or shades, etc. all contribute to heat gain.