How to tell if your co2 tank is 20 lbs?

I have a 20lb alum tank feeding my 40g planted tank, the cylinder portion is approx 23" x 7-1/2" . . . don't know what you're running with it but I have mine on a solenoid (10hrs on) and it just went into the red zone after 21 months . . . get a friend to knit you a nice cozy for your nuclear warhead.
 
AquariaCentral.com