How to tell if the Hydrolic Slave Cylinder or Clutch needing repair??

[OverRunWithSons]

I am the wife asking for my husband, we have an '88 Suburban with manual tranny, he did change the Master cylinder about a year ago or more in it. Now he thinks/hopes he only has to change the Slave cylinder on it. This is what has been happening.... Been slowing getting worse over a few months.... It changes into gears fine, it is when you go to put your foot on the gas, you can hear the motor raoring and lacking power, it does this with all the gears (I think). How to know when it is a Slave problem Vs. a Need a whole new clutch problem? Where should be start in fixing? Thanks, love this board for advice.