I'm thinking of putting a triple on my touring bike but only if I can use the shifter I have – I believe some shifters are OK for both double and triple whilst some are not.

disconect the cable and count the clicks

Well OK but how many clicks do I want? Is it 4 for a double including the trimming? So 6 for a triple?

9 or 10 speed?

10 speed – actually I guess it'll be 3 clicks for a double and 5 for a triple?

there should only be 2 clicks for a triple and 1 for a double (i think)

from the small chain ring to big = 1 click (double)
from small to middle (1 click) middle to big (1 click)= 2 Clicks (triple)

As far as I know all 105 front sti shifters are triple and double compatible.
However, sti double levers also have a 'trim' function that is a 'click'but it does not throw the mech as far as to change a chainring.

Original 5600 model 105 shifters did both double and triple depending on which front mech you used. The later models (released about 6 months ago) are double or triple specific.