There are three types of meteor detectors, the Meteor Crash Detector, the Meteor Time Detector and the Meteor Proximity Detector. They are all items that you hold in your hand unlike the Meteor Timer which you have to place down as a block.
NOTE: All detectors will scan around in random directions when there is no meteor/crash site to track currently. It acts just like a compass would if it was in the nether.
NOTE: Read the tooltips on the detectors! It will inform you of the current status of the detector.
Meteor Proximity Detector
The Meteor Proximity Detector will point towards the next incoming meteor that is closest to you. It may not be the next meteor to fall down, but it will certainly be the closest.
Meteor Time Detector
The Meteor Time Detector will point towards the next incoming meteor. So in other words, that's where the next meteor is going to land. To know exactly WHEN the meteor will fall, use a Meteor Timer.
Meteor Crash Detector
The Meteor Crash Detector will point towards the last crash site. (Due to some technical disabilities, it may change direction slightly when you get closer to the projected crash site. This is due to how meteors travel downward at a slant in a random direction. So when the chunk containing the meteor is loaded, the meteor will crash down and the location of the crash site will change. It shouldn't be far off though from where you were heading.)