The questions I ask really vary. Depends on what's being done. Also depends on who I'm talking to. A foreman for a huge company might not care if he tells you all the secret stuff they do at install. Where a company owner might guard it like a hawk.
One specific competitor of mine has enough pride for 2 people. I use that to his disadvantage. I'll see him on a job and challenge something I see. He'll go on and on about what he does, why he does it, just to prove he's right. That helps me in 2 ways - at bid time when customers are considering who to choose (and I can tell them how each company will approach this project), and at install time, so I can be sure I'm installing the best way I can.
Some people won't give you any info. Some will give you lots. You just have to play it by ear. Just approach it innocently, "I'm new in town and wanted to say hi. I'm sure we'll see each other all over town and just wanted to intorduce myself. And hey, whatcha workin' on there?"