what are two methods by which Santiago can tell where there's fish?

By watching the birds, particularly the man-of-war.

"Just then he saw a man-of-war bird with his long black wings circling in the sky

ahead of him. He made a quick drop, slanting down on his back-swept wings, and then circled again.

“He’s got something,” the old man said aloud. “He’s not just looking.”

I am looking for another, but the way Santiago runs his line and bait seems to be the only other thing I see here.