While watching Star Wars Episode I, I started thinking about how Darth Maul was searching for the Jedi (and Padme) with those probe droids. Is there anything in the Star Wars Universe to indicate that droids can sense how strong the Force is within someone? Something like a long-distance Midi-chlorian count, or anything along those lines.
I'd always assumed the droids had used visual profiles of Qui-Gon, Obi-Wan and Padme to search through Mos Espa - since they are all (to a greater or lesser extent) public figures, there'd be plenty of imagery of them available. We see when Qui-Gon tests Anakin's blood that Force sensitivity can be detected mechanically, but this requires physical contact to gain a sample.
In the Jedi Academy books, Luke (and Lando, if memory serves, though it could have been Wedge) find an Imperial 'Force detector' on Coruscant that can scan someone without making contact, though it's more like a handheld metal detector and would be difficult for a spy droid to do discreetly. You could speculate that if this had been available, the droids might have been fitted with them - but that's all it would be, speculation.
The implication from the film's official novelisation was that the DRK-1 droids were indeed looking for powerful force users. This explains how the droids were able to track Qui-Gon back to his transport.
He lifted his arm to view the control panel strapped to his forearm, picked out the settings he wished to engage, and punched in the calculations required to identify the enemy he was looking for. Jedi Knights would manifest a particularly strong presence in the Force. It took only a minute. He turned back toward his ship. Spherical probe droids floated through the hatchway, one after another. When all were clear, they rocketed away toward the cities he had identified.
It's possible that the author simply meant that Maul (as a force user) could tell that they were in the vicinity due to their force presence. Precisely what sensors the droids used to perform this 'Jedi search' aren't explained in any detail, nor is this particular technology mentioned in any other canon source which leads to the conclusion that this was an invention of the author and hence non-canon.