how do i know when the clutch ic bad? or transmission? i just bought a '91 mustang gt. the car starts but the clutch makes a poping noise when i engage it.

 

This could be a broken transmission or motor mount.It doesn't have to be but it could.

 

could be the throw out barring in the clutch

 

When the clutch is getting worn out the disc becomes
less thick, like worn out brake pads.
The pressure plate has a spring driven
plate that applies pressure to the disc.
When disc is worn the springs no longer
apply enough force and the transmission
slips.
From a stop, geer in 3rd, let out clutch and if rpm
goes up and car is not going I'd suspect clutch.
Sometimes when disc is worn enough the bearing that
depresses the pressure plate springs may travel a
lot further than normal and pop and bang from the
long travel.
I would look into adjusting the throw of the bearing
and see if it helps.

 

this might not help but when my tranny took a dump i could start the car and hold the clutch pedal down and nothing seamed wrong until i let the clutch out then i could hear the metal bouncing around in my tranny.