...besides, "What is that?"
1. How did you get into this scene?
Notice the difference between HOW and WHY. Asking WHY, generally speaking of course, is accompanied by a mocking, skeptical attitude, while asking HOW shows less bias and more curiosity. Whenever you come across a unique hobby or automotive enthusiast niche, you may find there was a likewise unique entrance into that "world."
2. What all is done to your truck, and who did the work?
From this question you'll be able to determine how "into it" the driver is. If he did it himself, this question alone could lead in to all you ever wanted to know about the vehicle and ten times more. If the work was done at a shop, you can find out about the shop, and if the owner's happy with the work. You might find out when this work was done, and how long he's had the truck. Some questions might include: What kind of valves and bags are you running? Any engine work done? How much air, what kind of compressors, how much pressure?
Custom cars and trucks end up at shows, or the owner may attend shows without showing his ride. From there, you can find out which shows, what classes the truck is in, and if the owner's won anything or hopes to one day.
There are many car and truck clubs out there, so if you see an unfamiliar sticker, this may be a club, and another thing to talk about. Asking what area the club is out of, how many rides, and what kinds of things they do as a club may be informative.
5. Do you have future plans for the truck?
Although it is possible for an owner to be completely satisfied and content with his ride just the way it is, often, trucks are a work in progress; there is always something else the owner wants to get done. Find out what these next steps are, and what kind of timeframe is planned out for the project. On the other hand, it is not uncommon to see mini trucks for sale, so finding out why it's for sale might be informative as well. The bottom line, when conversing with a minitrucker, lies in showing interest, instead of inadvertently passing judgment.
And finally, the BEST question that I've ever been asked came from a mom with stroller, who crossed the street in front of me. When the truck in front of me hit switches as she passed, she looked over without an expression, then as she passed my window, calmly asked, "C'mon! Aren't you gonna hit back?"