Conduction power or switching power?
For conduction power you can either look at the datasheet and see what the Anode-Cathode drop is at your current. Then a straight V*I
Or you can do it practically - measure the Anode-Cathode volts and the amps and multipy them.
If you want switching power/losses you will need to, using a scope fire the triac with a given voltage across it - this voltage will decay as the device turns on. Eqaully as the device turns on the amps will increase.
With these two rising/falling waveforms, they need to be integrated over the time it takes to switch
Joules = Int( V*I,t=0...ton)
Most modern scopes can do this integration on the scope, otherwide
specific points will be needed to get the best resolution.
Now multiply by your switching freq and you get the turn-on switching loss - do the same for turn-off and you have all the switching losses and conduction losses.
These are to be done at application-rated volts and amps.