Argie ... there's no real way to know without looking. Or running out of petrol !
On all the Triumphs I've had, the main tap is on the right hand side, with the reserve on the left.
If you wanna be sure, either take the taps out and check, or, if you donn't want to disturb them, drain the tank from the right hand tap, and then check to see if you get another few pints out of the left one ?
Good judgment comes from experience, but the most memorable experiences tend to come from bad judgment!