Examine your reasons for suspecting your refrigerator has a gas leak. If the refrigerator is simply not cooling properly, don't assume you have a gas leak. It is just as likely that your refrigerator is experiencing some other type of component failure. The idea that the gas in a refrigerator wears out or diminishes is a misconception. The gas exists within a closed system and is compressed. It does not naturally dissipate. Only a leak will cause there to be less gas.
Smell around for a possible leak. If you suspect there may be a refrigerator coolant leak, you should be able to smell it. Freon and HFC-134a have distinct odors that make them easy to detect. At the first sign of a gas leak, open up your windows and doors and stay away from the refrigerator. These gases are not known to have any long-term health consequences, but they can cause you some illness in the short term that you'll want to avoid. Both are heavier than oxygen, so the smell should remain low to the ground unless it is a significant leak that occurs quickly.
Unplug your refrigerator and use a soap-and-water solution to check for leaks. You can use a common dish soap and a little warm water to do this test. Fill a spray bottle with the solution, pull out your refrigerator, and spray some of the solution on the coils and around the compressor and its gaskets. If the gas is leaking, you should see the solution begin to bubble at the point of the leak. This test will not work on all refrigerators. In some refrigerators, these components are enclosed and not accessible.
Contact a qualified service technician. Determining gas pressure in refrigerators requires special gauges. Service technicians are licensed and certified to do this. Because of laws protecting the environment from the release of harmful environmental gases, you should leave the work to a person who is authorized to do it. Plus, if your refrigerator is not leaking gas, the technician will be able to determine what your refrigerator will need to function properly.