HOW DO I KNOW WHEN THE THRUST BEARING IN MY SLAVE

who told you that???? a wally or what?
im sorry to tell you the cluch slave cylinder has nothing to do with the cluch thrust bearing the slave cylinder only operates the cluch rod,, which inturn pushes on the cluch thrust bearing which then disengages the cluch,,,if the trust bearing is warn or wearing out it rumbles or starts a knocking sound from the gear box,, this noise gets worse as the cluch bearing wears out,, finaly it starts to realy screem,, bad like!!! the problem is,, you need a new cluch fitted to the bike which is far from cheep to get repaired,,,,,and its a big job too,
i hope this has put you right,,
good luck,,from1 biker 2 another