No easy way to know but what you can do is disconnect the line at the clutch master, and then buy a threaded plug (or even a bolt) to fit into the hole where the line came out. Then you fill the master, pump it up and bleed it by loosening up the bolt. If after a few bleeds you have a very hard (almost immovable) clutch pedal, then you know the clutch master is okay. If you can't bleed the clutch master enough to get a very hard pedal (after all, it's not pushing anything but the bolt) then you know the master is done for.
By the way, some hydraulic clutch systems are not easy to bleed by the conventional methods, and may require a pressure bleeder.
Also, if you noticed a fluid level drop before you bled the system, or any leakage at the slave cylinder, then you definitely have a bad clutch slave cylinder.