Well I guess the signal strength could be measured but clothes hanged in wardrobe are too close to each other to distinguish them. Maybe close range NFC?
I have other solution to be considered.
Maybe you look at the wrong touchpoint of that interaction. What if the desired action occurs not while it's already hanged but when user put the clothes on hanger? During that, it's not with other clothes (as you grab the hanger in your hand to put the clothes on it).
There are a few things to consider. I guess people put things only on one hanger at once while having clothes in second hand. How many % of users puts clothes on hanger by grabbing it outside of wardrobe (as mundane event/behavior)?
Why it's important?
During that interaction there is a moment when user tries to pair single cloth with single hanger separated from the rest.
The hanger could have additional sensor to know when it happens:
- Magnetic field sensor and small magnet in collar?
- Capacitive sensor?
...so the device will know when that interaction occurs and then activate the reading from single rfid tag.
I guess my answer is more about user experience and the aesthetics of interaction but it can give you some clues to finish your project.