Its not rocket science----or maybe it is. IF it doesnt look like a correctly bent tube, no twists, kinks, gouges, crushed areas, mangled flares, then it probably needs to be replaced. IF you are a novice on here, look for pics of properly made tubes. There are a bunch. If you are having trouble making them, seek help. There is alot of that here too.
Just because we use a CNC bender to make our production tubes, doesnt mean that we dont use a hand bender to make a concept prototypes, or even a difficult tube that our CNC dies wont accomodate. Like this one I did recently for a RV7A with an Aerolabs gascolator. 180* bend with the flare close to the start of the bend, then a 90* vertical leg, again close to the end of the original bend. Trick was all of this had to fit in a tight area.
Use a good bender, inexpensive is fine, but take your time and practice BEFORE you attempt to make a critical tube. I have a box of various bent tubes with different angled bends just for playing around. Its easy enough to see if something is doable, than to spend a bunch of time and waste tubing trying to make something that your tool cant do.
A quick word here about 3003O aluminum tubing that is provided in Vans kits. Its relatively inepensive, and soft enough to be bent by hand, given some circumstances. BUT that doesnt mean you can kink it and then go fly. It does take some care to do properly. If you arent sure, seek some guidance
from a tech counselor, or an A&P, or even your hangar buddies. A fractured fuel line under pressure in flight is a bad day waiting to happen.
Last note---INSPECT THEM. Make sure they have proper supports and cant contact a structure or something else to cause potential damage. We've seen alot of them, even on certified planes.
Tom