To check for leaks, the operator will need some gas that can be sprayed in small areas (using a small nozzle, such as disposable pipette or gas welding torch). Argon is a good leak detection gas, but others can be used as well.
To check for leaks with a detecting gas, spray a small amount of gas in as small an area as possible. Focus on areas around connectors and seals, especially those that have been recently disconnected for maintenance. Do not forget the column nuts and septum/inlet area, but keep in mind if you spray a leak on the INLET end of the column, there will be a minute or so delay before the gas gets to the MS chamber.
Patience and working systematically is the key to finding high vacuum leaks, so work slowly and follow a pattern. Spray a small area, and wait for the signal; unless you are spraying the INLET end of the column, the signal should appear nearly instantly, but certainly within a second or so.