The vegisir (boss marker) stones for Yagluth can sometimes be found in Fuling Towers and Fuling villages. Also, the Stonehenge structures sometimes have them, as do occasionally just a couple of standing stones next to each other.

As others have pointed out, oftentimes one finds the altar by spotting the distinctive stones of the site. There will be several that look like somewhat like the typical monoliths, except they will be arrange in a circle and the pillars will be curved as they go up, much like a grasping hand. If the weather is clear, they can oftentimes be spotted from quite a distance, so sailing along the Plains in good weather is one way to find them. I have done several run-throughs of this game and more than half the time I find the altar before I find a register stone.

All that said, it can take several hours to finally find Yagluths altar. It can be a pain to sail around for days, looking for it while also checking to spot any Stonehenge or standing stones that may have a register. So hang in there.

Cheers!