How internal crack propagation can be tracked and rectified especially in LPG lines?

by performing a thorough leak test we would know the fault where it was leakage

Through Leak testing method 

or 

Soap water method 

NDT test is the most basic procedure to determine any defects or cracks on the pipeline.

we can check by RT Level 2 in x ray

In the first place, the crack must first be localized and delimited by using the NDT test by penetrant or X-ray, after a suitable welding process has been established,

the inside crack on the LPG pipelines can be tracked by using Magnetic Flux Leakage pig or Transverse Flux Leakage pig inside the pipeline.

With my knowledge you have to go NDT test to identify any crack or fraction on inside surface but you can use N2(g) to check are there any leak existing along the pipe line or flanges and then if found use detergent/soup to identify exact place where leaking.