I agree that a pair of thread gauges are essential.
I'm sure that most of the makers on here can tell common thread pitches at a glance.
If you're out and about and you didn't happen to bring your thread gauges, you can download the free Thread Pitch app to your smart phone and do a quick thread pitch check. It won't replace your thread gauges, but it works well enough to show your customer that his 3/8-11 pin is actually a 3/8-10.