Although the EMS soft starter is getting very old, there are still many giving good service in industry.
To check the performance of the starter, I always begin by connecting three phase on input and three 300 Watt star connected lamps on the output, star connected with the star point isolated from other circuitry. i.e. a three wire circuit.
All three lamps should ramp up evenly to full brightness.
Using a multimeter, ensure that there is full line voltage across each lamp.
Because there is the potential for one SCR to be lazy, measure the voltage forward and reverse.