This ground is constantly being recovered in lava so there's no signs. There's a trail of yellow studs in the ground for the first third of a mile (presumably studs are easy to drive into the ground and so cheap to replace), then the National Parks service put yellow flashing lights at high points every third of a mile from there. When you get to one flashing light you look out and try to spot the next one blinking away in the distance. Of course, as soon as you step down you loose sight of the next beacon, so you have to keep scrambling up the rocks to get sight of it again and so ensure you're still heading in the right direction.
The last 45 minutes of the hike is beaconless. You just keep going, but by that point you have another guide - heat. It gets steadily hotter and hotter until you can eventually see the lava glimmering in the distance.