You can use a lot of methods.
It´s necessary to know some details, like:
- Use: requeriments are different depending of the use. Power plant pipng has higer requeriments that water-treatment plant (pressures, tempratures......)
- kind of joint: For groove welds you only can aplly RT and UT testing (volumetrics). If don´t need a complete penetration, you can use MT or PT (surface).
- Material: In non ferrous material (aluminium or PE))you can not carry out MT testing. I imagine tube is manufactured in steel (maybe ASTM A-106)
- Thickness: With 0,32 inch. or less you can not use UT testing.
- Leak testing is indicated to asure watertightness
If that tube hasn´t high requeriments, PT is the best.