Well some RO units come with a PSI gauge showing the pressure going into the unit. You may be able to buy a aftermarket gauge, but I'm not sure who would sell one.

Where did it crack? Close to the threads or on the side or on the bottom? Sounds like it may have cracked at the threads if you just replaced a filter. If it was overtightened, that can lead to stress and that could make your cannister crack. OR it could have cracked when overtightened and it finally gave way now.

Just make sure to check the main unit also. It may have also cracked there too. You don't want to order a new cannister only to find out you have to replace the main unit also.....