Drop in drills to find the thread core size and consult thread tables. Likely Whitworth, depending on the age of the machine.
I would suggest you consider a QCTP if you do more than a minimum amount of turning. I bought one, but they are fairly easy to make if you have a reasonable milling machine or slide (for lathe). I got fed up with keeping adjustment shims for each cutter. Best single improvement I have acquired, I reckon.