I believe there have been many ways/devices/means made/used over the years that are capable in various manners (including various gas, hydrostatic, and even ultrasonic etc. means) of individually testing "flanges" and joints, and even from the "outside". However, while there is perhaps utility to all these approaches/devices in some circumstances (particularly e.g. for diagnostic purposes prior to filling a line and more extensive or difficult testing) in my opinion they are in general not technically equivalent e.g. to an eventual, properly run hydrostatic test of the entire pipeline.