If you really want to know the thickness of the tubes and don't want to cut them up to find out then you'll need an ultrasonic thickness gauge. You can buy them on Amazon but I have no idea how accurate they are as I've never used one.
Alternatively, one of my ex-employers make a tube wall thickness gauge that used Gamma rays from a 10 Curie Caesium 137 source shrouded in depleted Uranium and Lead. I can vouch for the fact that is very accurate but it's rather expensive at around £500,000 and you'd have to be prepared to wait about 2 years for them to deliver one - 10Ci Cs137 pellets are rarer than rocking horse manure. Then there's the issues of licensing its use, somewhere safe to install it, a certified Radiological Protection Advisor to okay the installation, etc, etc, etc, ... the problems are endless. Personally I'd go for the Amazon special
