Master or Slave Clutch Cylinder?

A couple of weeks ago, I noticed that even with the clutch pedal fully depressed on my car (2001 Daewoo Nubira CDX 2.0L DOHC) my reverse gear would grind as though the clutch wasn't fully disengaged. As the week progressed, I noticed the pedal becoming spongier until one day it dropped to the floor and wouldn't come back. Me and a friend bled the clutch line (air came out), and up until today, it has worked fine, but now it is getting spongy again. I found that vigorously pumping the clutch will cause it to stiffen up enough to make it driveable, and after the first drive of the day, It is less spongy every other time that I use it; and when I'm done driving, it feels completely normal. I've inspected all of the clutch line and have found no evidence of leakage, so I'm fairly certain that one of the cylinders is bad, but I'm not sure which it is. Are there any ways to determine whether it is the slave or master?