To test the health of a triac we can use a multimeter. Step 1. put the selector switch in a high resistance mode (say 100K), then connect the positive lead of multimeter to the MT1 terminal of triac and negative lead to the MT2 terminal of triac (there is no problem if you reverse the connection).The multimeter will show a high resistance reading (open circuit). Step 2. Now put the selector switch to a low resistance mode, connect the MT1 and gate to positive lead and MT2 to negative lead. The multimeter will now show a low resistance reading (indicating the switch ON). If the above tests are positive then we can assume that the triac is healthy. Anyway this test is not applicable triacs that require high gate voltage and current for triggering.
Step 1. Test the TRIAC
Step 2. Test the TRIAC
Step 3. Test the TRIAC
That's all. Have a nice try.
reference: http://www.circuitstoday.com/how-to-test-a-triac