I think when I offered advice, I started out with ,
"Hi I'm sorry to intrude, but I notice you seem to be battling with the camera for control of the flash? Would you like a little tip to give you the upper hand?"
It seemed to work, a little lighthearted humour seemed to work, of course the pop up flash is probably the easiest subject to tell they could use a tip!
I'm glad to see that some of you would offer unsolicited advice, I know if I was struggling and someone offered me advice I would gladly accept, I doubt I would ask for fear of them being unappreciative of the interruption!
I have no problem helping complete strangers. I see offering some unsolicited photography advice much the same as I see holding the door for someone, picking up something the dropped and bringing it to them, etc. You're just being polite. And I think as long as you do it in a nice way there shouldn't be any problems with practically anyone. Saying something like: "hey, that's a nice camera. I'm a bit of a tech geek, do you mind if I ask what model it is? **they answer** Oh cool! I've heard a lot of good things about that model - what do you think of it?" That right there will usually uncover frustration on their part. Then I'd say something like "if you're interested, I might know a way to help". If they say no then I'd respond with something like "yeah, like I said, I'm a tech geek so I don't like people touching my stuff either" then I'd smile and tell them to have a good day. If they take offense well... that's a reflection on them, not me. But more than likely, you're going to end up helping them.