Doing leak check using soap suds is a very standard way of checking for leaks in piping pressurized with air or gas.
Refer the link below on some tips related to using soap suds or soap solution:
http://www.hvacfun.com/a-leak-detection-methods-equipment.htm
Also refer Section 7.6 of the the link provided below:
http://boonchoo.org/OCS4/2.5%20Piping/2.5.1%20Engineering%20Specification/SPC-0804.02-50.06%20Rev%20D2%20Fabrication%20Inspection%20and%20Testing%20of%20Piping.pdf
A gas detector may also be used if the system is pressurized with process gas or hydrocarbon gas as per the link provided above.
Regards,
Ankur.
