If the clutch master is leaking, you should be able to see fluid escaping, most likely where the rod goes into the back of the m/c inside the car. I don't know if there is another seal on the rod itself to stop that escaping fluid leaking out - you'd probably smell it too, especially if the heater was in use.
If the slave cylinder is leaking, same same, even a smell of fluid perhaps (if taking a close look). It is usually obvious, because the escaping fluid catches the dirt, but if there is a rubber plug over the end of the slave, it could have a drain to allow the fluid to escape with little to show. See if you can dislodge the rubber plug, and check for moisture inside.
Either way, if there is a leak you should be seeing the reservoir losing fluid - have you had to top it up? If it has been bled, is not losing fluid, has been adjusted, the problem would appear to be on the other side of the slave - up to and including the gearbox. In which case, it might be time for a professional to have a look?
