Assuming that pumps and gauges are routinely maintained and performing correctly, a detailed leak test of the system will be required.
There are many methods of leak detection, such as a bubble test or pressure rise test, but none can compete with the sensitivity and ability to pinpoint a leak that a helium leak checker can offer. Additionally, a helium instrument allows for both quantitative and qualitative measurements.
A helium, or more precisely, a Helium 4 leak detector is essentially a tuned mass spectrometer which detects this gas and has the further option of identifying Helium 3 (used in cryogenics) and hydrogen. The unit has appropriate pumps supplied, making it portable, with only mains electricity required. The unit is extremely sensitive to helium. For high vacuum applications a leak rate below 10-6 mbar l/s is needed, and Helium leak checkers have an ultimate sensitivity of 10 mbar l/s.