^generally when you've been welding long enough you can tell just by looking at it. you inspect to check the penetration is ok, look to see if there's any contamination (you've really fucked up if there is) a lot of it falls down to the prep work, if you look in the pictures you will see where 2 pipes meet the edges have been preped for welding, in simple terms a 45degree edge has been ground creating a "V groove" where the 2 pieces meet, which is perfect for butt welding
if you want to get uber techy, then you can get into all sorts of testing, dye penetrant, izod, xray blah blah blah, not really any need for this application