How can you tell if a head gasket has gone??

My mtx75 gearbox is leaaking no idea why changed the slave cylinder and cant see any cracks inside it but it drips from the drain off at the bottom of it..

Car is a Ford focus 2.0l

 

A good garage will have a tool that can measure if CO is present in the coolant.
When present the head gasket is gone.

 

if the head gasket is gone then do as above, signs are losing water but it aint leaking, mayo around the oil filler cap, sometimes mayo round the water filler cap. and in extreme cases. water pissing out of the exhaust or if ya take the oil dip stick out an its like mayo.. also lots of pressure in the coolant tank. but for the love of god don't take the coolant filler cap off when its hot, me the silly person did it when i was 17 and ended up in hospital with burns to my arms and hands.

 

Sniff test mate

 

None of these presant. Just gearbox problem

 

My MTX75 leaked when it was over-filled.

 

Leaked when over filled eu. From the drain off at the bottom of the box??