Take the cap off the air valve at the top of the tank. Press in on the valve core and if water comes out, the bladder is most likely shot. It can be changed, but you’ll be lucky to find a replacement without having to order one. Most of the box stores that sell the pressure tanks don’t even think about repair parts. Our ‘throw away’ mentality says it’s easier to just replace the tank.
If no water comes out of the valve core, it could just be low on air. But, big but, you need to drain the tank of water before shooting more air in so you get the proper amount of air pressure to coordinate with your pressure switch. There should be a label on the tank telling you how much air pressure the tank needs to operate properly with the switch.