From 1,000' below the couple of oktas of cumulus within ten or twenty miles of Howland, she'd have seen the massive smoke signal from Itasca quite easily. Sure, the island would have been a bugger to spot from much more than five to ten miles away, but she knew that. It was the ship and its huge smoke plume that she was looking for. That's a very different visual cue from an island which looks like a cumulus shadow.
Here's a snippet I found when researching in the Purdue files. She wrote a first draft of her memoirs in the interregnum between her first and second RTW attempts. Here's what she said of her solo flight from Hawaii to California:
