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This is also the impression conveyed by the The Empire Strikes Back: So You Want to Be a Jedi?, in which Vader can "sense" the presence of Luke and the Rebels, as mentioned in another answer.

This is also the impression conveyed by the The Empire Strikes Back: So You Want to Be a Jedi?, in which Vader can "sense" the presence of Luke and the Rebels, as mentioned in another answer.

In that vein, the size of the base might be a clue as well that there is something more than smuggling going on. Also, the picture in the film appears to be of shield generators, which should be a dead giveaway that it is a military facility.

Of course, Luke Skywalker was also at that base (which he was not in the previous case), and Force users have been shown to be able to sense the Force in others, albeit inconsistently.

In that vein, the size of the base might be a clue as well that there is something more than smuggling going on. Also, the picture from the probe droid shows the power/shield generators quite clearly.

The monitor displayed the transmitted image of a snow-base power generator.

—The Empire Strikes Back (Junior Novelization)

These may well have been of the sort suitable only for a military installation, as opposed to a smuggler based unused to combat. Certainly if they could be recognized as shield generators, it would seem likely that the occupants of the base were more than mere smugglers.

Of course, Luke Skywalker was also at that base (which he was not in the previous case), and Force users have been shown to be able to sense the Force in others, albeit inconsistently.

VADER: That is the system. And I'm sure Skywalker is with them. Set your course for the Hoth system. General Veers, prepare you men.

This serves as further indication that it was Luke himself that Vader sensed.

True, Darth Vader's skill in the Force was not enough to simply pull the location of the base out of thin air. But it is possible that, with additional information (such as a picture of the Rebel base), the Force could provide him with sufficient intuition to know whether it was the right place or not.

This is also the impression conveyed by the The Empire Strikes Back: So You Want to Be a Jedi?, in which Vader can "sense" the presence of Luke and the Rebels, as mentioned in another answer.

True, Darth Vader's skill in the Force was not enough to simply pull the location of the base out of thin air.

As Admiral Motti says:

ADMIRAL MOTTI: Don't try to frighten us with your sorcerer's ways, Lord Vader. Your sad devotion to that ancient religion has not helped you conjure up the stolen data tapes, or given you clairvoyance enough to find the rebels' hidden fort-....

But it is possible that, with additional information (such as a picture of the Rebel base), the Force could provide him with sufficient intuition to know whether it was the right place or not.

This is also the impression conveyed by the The Empire Strikes Back: So You Want to Be a Jedi?, in which Vader can "sense" the presence of Luke and the Rebels, as mentioned in another answer.

Of course, Luke Skywalker was also at that base (which he was not in the previous case), and Force users have been shown to be able to sense the Force in others, albeit inconsistently.