I'm trying to measure the Q-factor for a set of Speedplay Ti pedals and a set of Time RXS carbon Ti pedals.

What is the best way to measure this? should I measure to the center of each pedal?

Q more applies to the crank..

Take off the left crank, reverse it on the spindle 180
so both arms are facing the same direction.
then measuring will be simple..

How much variation is there in pedals? they all have to accommodate the width of a shoe. You can measure from the flat face at the crank to the centre of the clip mechanism.

I've held a ruler across the chainstays with one hand and slowly turned the cranks with the other. If you're behind the bike and able to line up an eye behind the pedals you can get a pretty good measurement. I doubt there's much difference between pedals, but like cranks it's the kind of measurement I'd like to know.

The OP is asking about pedal width, not crank width, but I'll chime in with my method for measuring the crank Q-factor. This is easiest with a caliper but a ruler works okay as well:

Line one crank up with the seat tube and measure from the outside face of the crank to the tube, then do the same with the other crank, then measure the diameter of the tube. Add all three numbers together and there you go. With three separate measurements there is a little more chance for error, but I can usually repeat the measurement within 1-2mm so close enough.

This is the correct answer to the OP's question.

I have been wondering about a related topic. I am interested in Tioga Spyder or Surefoot pedals but the axles look awfully long...as if they'd substantially increase the q-factor (I prefer a low or moderate tread). Is this a correct assessment or are the pedal axles actually not as long as they appear?

With those platform pedals, where you put your foot is different every time, as there is no retention mech.
Appears the platform is relatively small in proportion to the given 9/16" pedal mounting thread.

As has been alluded to, the Q-factor is the width of the cranks, which is independent of the pedals. https://sheldonbrown.com/gloss_q.html#qfactor

I think the Q-factor includes the crankset and pedal spindle as described by MichaelW, above. I think the term "Q-factor" originated from the medical term "Q-angle", the angle from the femur to the tibia with the leg extended.

This is a pretty important measurement for some, maybe a lot of people. It's too bad it's often not given. I think the most important measurement is between the outsides of the crank arms at the pedals, but I'd like to know of pedal width differences too.
I was recenty looking into sationary excercise bikes which often have very wide q factors,often 8-9 inches compared to 6 which I think is pretty normal for a bike. Some makers advertize a low q factor, but I finally gave up trying to ask anyone to give the actual measurement. I'm used to older road bikes and don't like to go over 15cm, or about 6 inches q.

Measure from the center of the pedal platform to the flat shoulder of the spindle which would butt up against the outside face of the crank arm. This will only give you the "Q-factor" for the pedal. The real life Q-factor for your on-the-bike setup would be measured from the center of your pedal platform to the center of either the down tube or the seat tube.

If you're interested in a particular new assembled bike, which often don't come with pedals, should you be able to ask for and get an accurate q factor measurement without a lot of trouble or uncertainty by just saying "q factor"? Is it a generally known measurement, or way of measuring?