I just hold a small piece of ordinary paper up to the gears, turn them so it eats the paper between them against hand pressure forcing them together, tighten things down, and then run the paper out. It has worked just fine every time. A slight bit of lash is all you need, though you do have to watch out for out of round gears, like savarin was saying, closing the gap. There should always be noticeable lash in the gears. Excessive lash is a bad idea, it makes the gears weaker and causes abnormal wear. I have seen some WAY loose gears on lathe change gear trains.