how can you tell if the clutch master cylinder is bad or the clutch slave cylinder is bad?

The slave cylinder would ususally (almost always) leak into the bellhousing if it has failed. If you are sure all the air is out of the system you can try disconnecting the line from the slave cylinder and step lightly on the clutch to see if it tries to force fluid out of the line. If I recall correctly it has a quick connect fitting and it should self seal when disconnected. If you have the type that seals shut when disconnected, the seal should prevent you from pushing the pedal down. If the line is sealed at the end but the pedal goes down you have a bad clutch master cylinder. WARNING: Do not clamp the line and pump the clutch while the slave is connected or the slave will push all the way out and blow the piston out. This will be followed by a rush of fluid from the belhousing. oops!

Well the real answer you need is that when one goes bad, like starts not developing pressure and leaking, if you change it, the other one will soon go bad too. This is because the new master either develops a higher pressure or the new slave requires a higher pressure to operate. Just change them both at the same time. Make sure you flush the out fluid reservoir and line with new fluid too.

Years ago I was stranded out in the desert due to a bad slave cylinder. The fluid had leaked out. All I had was some 30 weight motor oil. I put it in and it worked to get me home. I had to do extra flushes to get it the motor oil out of the system, but it was worth it.

We check for fluid leaks . Look under car if we have a puddle or wet fluid run from master cylinder will be in footwell or under firewall bulkhead.

If slave will be leaking from bell housing area.

Even lack of operation ought not to stop us putting it in gear when engine not running though possibly stiff. If no fluid leaking them master cylinder will not need topping up. So check that first.

If we have case of leak we can trace and have fixed, if no leak? Then mechanical issue eg gearbox selector? Linkage? Clutch actuating arm?

I had a defective clutch slave cylinder i(National Seal/Bearing BCA 619002) installed at Aamco in Santa Clara, CA. A few days later, the jeep became disabled on the freeway offramp. It had to be towed twice because the shop was closed on Saturday afternoon. I had to rent a car for four days and waste alot of time delaying with the busy shop. They even used by jeep to pick up parts and left a decaying apple core on the console. They will not reimburse me for the out of pocket expenses and will not give me the name of their distributor.

I have the defective part.

Any recommendations on how to proceed?

It will be almost impossible to put it into gear withou it grinding. Also the clutch pedal will stay to the floor sometimes, also if you can pump the clutch pedal until it get a little hard and then you can put it in gear with no grinding then you have a bad clutch cylinder, Have you checked the fluid in the cylinder if it is low it will not go into gear.,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

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The way you describe the problem, it can only be the master cylinder not returning all the way. This can be due to swollen seals or maybe not enough free play at the pedal. Most likely swollen seals or broken/ binding return spring.