Vader sensed it (with the Force). He also noted the presence of another powerful Force user; Luke Skywalker.
“It is they,” he intones. “The rebels are there.”
“Lord Vader,” objects Admiral Ozzel, distinctly displeased at being contradicted, “it could be anything. Smugglers, a small settlement…”
“It is the rebel base. I can sense it. And Skywalker is with them.”
“Lord Vader…” Admiral Ozzel does not put much stock in Vader’s “sense,” nor in the hocus-pocus of “the Force” and “the dark side.” Admiral Ozzel is a sensible man. A man of science and warfare. - The Empire Strikes Back: So You Want to Be a Jedi?
