The well pump we have is over 15 years old.
The pressure is fine but it's started to rust near the bottom due to condensation.
How can I kow if the membrane on the inside is still ok or if it has ruptured?

 

If the bladder ruptures the pump will short cycle. Check or set the tank charge 2-3 psi below the pump cut in with the water pressure released. See if the system cycles correctly.

 

I am not clear what you have. There are pumps that have a membrane or diaphragm but they are rare for a home but are common on campers (RV, caravan). You may also have a separate pump and a pressure tank which contains a diaphragm. Which do you have?

 

It's a well-x-trol - red pressure button on top.
There are no pressure problems and the pressure switch only ticks occasionally (as normal).
However, when you hit the side of the tank, there is no hollow sound, it is almost solid (meaning full of water)?

 

A pressure tank with a bladder can last a long time. Then when the bladder eventually ruptures the tank works like an old fashioned bladderless tank.

I'd bet your pressure tank is still OK. To check it turn off the circuit breaker for your pump. Open a faucet or spigot and let it run until no water comes out. Then use a tire pressure gauge and check the tank's air pressure. There should be a fitting like on a tire at the top. If there's air inside (probably something between 30 and 50 psi) then your bladder is still OK and holding air. If you have no air inside then your bladder has probably already ruptured but you can use a compressor to add air and see if it holds it.

 

If it ruptured, then wouldn;t we be drinking rust?
The ewater pressure is at around 30psi most of the time, there's a little guage connected to to the tank -is that the same thing?

 

Rust, iron oxide is not harmful so I wouldn't freak out unless your getting chunks caught in your teeth.

To verify if the bladder is intact you have to check the air pressure while the water system is completely discharged. Most of the time the air pressure will be the same as the water pressure with or without a bladder or a failed bladder. If you've not touched the system for a long time and you're not complaining about water pressure fluctuations then I'd bet the bladder in your tank is OK.

You may have gauges in two locations. Most commonly a gauge is installed in a water line near the pressure tank or switch. A gauge can be installed in the air portion of the pressure tank, though it's rare.