"Hey guys, how's the campaign going?"
"Give me some feedback!"
"FEEDBACK NOW, PUNY MORTALS, OR I WILL SMASH YOU!"
It can be helpful to be specific, though, as you'll get better answers. Ask about...
1. Pacing
2. Themes, motifs and tone
3. Ratio of combat/drama/investigation.
4. Style- I'm often told I don't pay enough attention to the rules, that I'm too fast and loose. So I try to keep this in mind.
5. Physical setting of the game and scheduling. (This matters!)
6. Character development and player "screen time."
7. Plot and NPC feedback. Find out if they hate the villain as much as they should, if they care about the damsels, if they want to know who killed Bob, etc.