Seriously?!? It's a survival site here. So here's the surivival tip: don't freaking touch them. If someone's near them, oh well. Sorry charlie. Good luck. I'll collect your corpse later. No point adding to the body count. Yes, it's harsh. But rescuer safety is priority 1. If you're "boxed in" then either A) stay inside the box, or B) you'll be that guy I just wrote about.

I've always heard live power lines spark/snake across the ground.
All I know is every downed line I saw at work, we waited a couple poles away, called dispatch with the pole number (where we were, not the downed one!), and waited til the power/phone company came out and THEIR guy touched the lines to prove them safe.

But, to answer the OP's line, sure there's a way to tell if it's a live line. Have something living touch it. If that living object dies, it's a live wire.