Generally hot plates do not come with a temperature indicator. There are more expensive models available that may have temperate controls but these are not necessary to invest in. The indication is when there are tiny bubbles appearing when the water is being heated, the beakers that contain the oil and water phases can be placed in. From herein, this is where the temperature of both phases must be monitored. It is important to note that you are measuring the temperature of your formulation rather than that of the water bath/bain-marie itself.
A thermometer probe can be used for both phases (one probe for each phase) which can measure the temperature accurately. An easier method is to use a laser thermometer where it does not need to be immersed but using laser pointer measurements. It works well for the water phase however when measuring the oil phase, it needs to be stirred when the laser pointer is aimed. If it is not stirred, the laser thermometer will only be taking the surface reading of the oil phase and not the actual temperature within the oil phase itself.