Master or slave? How do I know which one it needs?

I have a 2000 ford explorer sport,
today something happened to my
shifter or clutch maybe. I went to the
store, it shifted fine, I parked. When I
went to leave, I took it out of first &
tried putting it into reverse but it will
not allow me to shift into ANY gears,
im stuck in neutral! clutch pedal feels
very loose as well. someone said it
may be a slave cylinder, is that true?
what is your guess? a few people
have told me that it is probably the
master or slave cylinder. it is a
standard, yes it is hydrolic, no it will
not change gears at all, whether the
truck is on or off. .... can someone
please tell me how I can tell which
one needs to be fixed, the slave or
master?