I have a a '93 California Cooperage Coleman spa. When i give power to it and the thermostat is set, the low flow pumps kick on, but the heater light does not come on and the water stays cool. Could this possible be a pressure switch problem and if it is how can i test it?

could be a few problems

Pressure switch... check by a multitester across the two terminals when circulation pump is running...should be no resistance

Power relay... easiest way to check is to see if there is power at the heater element.

Heater element... if there is power to it, and no heat, replace element.

The order of things is the thermostat calls for heat, sends power to the circulation pump and the flow switch. If the flow switch detects the water flow, it sends power to the heater relay, which then pulls in. Once the relay is pulled in, it connects the heater across the line voltage, and thus, heat.