SCR consists of three pin of Gate (G), Anode (A) and Cathode (C). In order to identify the pin out, one must find it from semiconductor data book such the famous Philips ECG master semiconductor replacement guide. The data book will list out the general specification of the SCR such as the volt and ampere. If you want to know more details about a particular SCR, you can always try to search from the internet. Usually the SCR manufacturers will provide the full datasheet for those who want it.
Once you know the pin outs of the G, A and C legs you can begin to test the SCR. If you have the Peak electronic atlas component analyzer tester, what you need to do is to connect the three small clips to each pin of the SCR (any part number will do). The tester will begin to analyze the SCR and prompt you with the display such as "Sensitive or low power thyristor" before it tells you the exact pin outs of G, A and C. After the first test, the tester will eventually show you the answer at the LCD display. Red is Gate, Green is Cathode and Blue is Anode. It is a simple process and you will know the answer in less than 10 seconds. If there is a problem in the SCR, the tester would not be able to show the results instead it shows a shorted reading.