Any hydraulic clutch gurus here?

[Blackstone]

I haven't seen the truck yet, but it's a '96, 2wd, 4-cyl with an internal slave cylinder.

The owner says he's been losing fluid, and needs to pump the pedal several times to get the truck into 1st gear. My first thought is that either the master or slave cylinder is leaking. I'm first going to check for leaks, but just in case I can't find one... Is there any way to determine which item has failed without actually spotting the leak? Would the system behave the same way regardless of which part is leaking?